constellation: Stop assuming that the viewport is shared by all WebViews (#36312)

The `Constellation` previously held a `window_size` member, but this
assumes that all `WebView`s have the same size. This change removes that
assumption as well as making sure that all `WebView`s pass their size
and HiDIP scaling to the `Constellation` when they are created.

In addition

- `WindowSizeData` is renamed to `ViewportDetails`, as it was
holding more than just the size and it didn't necessarily correspond to
  a "window." It's used for tracking viewport data, whether for an
  `<iframe>` or the main `WebView` viewport.
- `ViewportDetails` is stored more consistently so that conceptually an
  `<iframe>` can also have its own HiDPI scaling. This isn't something
  we necessarily want, but it makes everything conceptually simpler.

The goal with this change is to work toward allowing per-`WebView` HiDPI
scaling and sizing. There are still some corresponding changes in the
compositor to make that happen, but they will in a subsequent change.

Testing: This is covered by existing tests. There should be no behavior
changes.
Fixes: This is part of #36232.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

26 files changed, 272 insertions, 256 deletions

diff --git a/Cargo.lock b/Cargo.lock
index a099a839bd6..634151d2bc6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6433,6 +6433,7 @@ dependencies = [
  "base",
  "canvas_traits",
  "constellation_traits",
+ "embedder_traits",
  "euclid",
  "fnv",
  "fonts",
diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index 90a7bb24cd1..62c9225876a 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -21,13 +21,13 @@ use compositing_traits::{
 };
 use constellation_traits::{
     AnimationTickType, CompositorHitTestResult, ConstellationMsg, PaintMetricEvent,
-    UntrustedNodeAddress, WindowSizeData, WindowSizeType,
+    UntrustedNodeAddress, WindowSizeType,
 };
 use crossbeam_channel::Sender;
 use dpi::PhysicalSize;
 use embedder_traits::{
     Cursor, InputEvent, MouseButtonEvent, MouseMoveEvent, ScreenGeometry, ShutdownState,
-    TouchEventType,
+    TouchEventType, ViewportDetails,
 };
 use euclid::{Box2D, Point2D, Rect, Scale, Size2D, Transform3D};
 use fnv::FnvHashMap;
@@ -479,6 +479,17 @@ impl IOCompositor {
         }
     }
 
+    pub fn default_webview_viewport_details(&self) -> ViewportDetails {
+        // The division by 1 represents the page's default zoom of 100%,
+        // and gives us the appropriate CSSPixel type for the viewport.
+        let hidpi_scale_factor = self.window.hidpi_factor();
+        let scaled_viewport_size = self.rendering_context.size2d().to_f32() / hidpi_scale_factor;
+        ViewportDetails {
+            size: scaled_viewport_size / Scale::new(1.0),
+            hidpi_scale_factor: Scale::new(hidpi_scale_factor.0),
+        }
+    }
+
     fn set_needs_repaint(&self, reason: RepaintReason) {
         let mut needs_repaint = self.needs_repaint.get();
         needs_repaint.insert(reason);
@@ -1248,13 +1259,13 @@ impl IOCompositor {
     ) {
         // The device pixel ratio used by the style system should include the scale from page pixels
         // to device pixels, but not including any pinch zoom.
-        let device_pixel_ratio = self.device_pixels_per_page_pixel_not_including_page_zoom();
-        let initial_viewport = rect.size().to_f32() / device_pixel_ratio;
-        let msg = ConstellationMsg::WindowSize(
+        let hidpi_scale_factor = self.device_pixels_per_page_pixel_not_including_page_zoom();
+        let size = rect.size().to_f32() / hidpi_scale_factor;
+        let msg = ConstellationMsg::ChangeViewportDetails(
             webview_id,
-            WindowSizeData {
-                device_pixel_ratio,
-                initial_viewport,
+            ViewportDetails {
+                size,
+                hidpi_scale_factor,
             },
             WindowSizeType::Resize,
         );
diff --git a/components/constellation/browsingcontext.rs b/components/constellation/browsingcontext.rs
index 6f8b24dabcb..0a6974a1f2a 100644
--- a/components/constellation/browsingcontext.rs
+++ b/components/constellation/browsingcontext.rs
@@ -5,9 +5,8 @@
 use std::collections::{HashMap, HashSet};
 
 use base::id::{BrowsingContextGroupId, BrowsingContextId, PipelineId, WebViewId};
-use euclid::Size2D;
+use embedder_traits::ViewportDetails;
 use log::warn;
-use style_traits::CSSPixel;
 
 use crate::pipeline::Pipeline;
 
@@ -50,8 +49,8 @@ pub struct BrowsingContext {
     /// The top-level browsing context ancestor
     pub top_level_id: WebViewId,
 
-    /// The size of the frame.
-    pub size: Size2D<f32, CSSPixel>,
+    /// The [`ViewportDetails`] of the frame that this [`BrowsingContext`] represents.
+    pub viewport_details: ViewportDetails,
 
     /// Whether this browsing context is in private browsing mode.
     pub is_private: bool,
@@ -85,7 +84,7 @@ impl BrowsingContext {
         top_level_id: WebViewId,
         pipeline_id: PipelineId,
         parent_pipeline_id: Option<PipelineId>,
-        size: Size2D<f32, CSSPixel>,
+        viewport_details: ViewportDetails,
         is_private: bool,
         inherited_secure_context: Option<bool>,
         throttled: bool,
@@ -96,7 +95,7 @@ impl BrowsingContext {
             bc_group_id,
             id,
             top_level_id,
-            size,
+            viewport_details,
             is_private,
             inherited_secure_context,
             throttled,
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index 8ef33132966..facfdff3885 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -111,7 +111,7 @@ use canvas_traits::webgl::WebGLThreads;
 use compositing_traits::{CompositorMsg, CompositorProxy, SendableFrameTree};
 use constellation_traits::{
     AnimationTickType, CompositorHitTestResult, ConstellationMsg as FromCompositorMsg, LogEntry,
-    PaintMetricEvent, ScrollState, TraversalDirection, WindowSizeData, WindowSizeType,
+    PaintMetricEvent, ScrollState, TraversalDirection, WindowSizeType,
 };
 use crossbeam_channel::{Receiver, Sender, select, unbounded};
 use devtools_traits::{
@@ -123,7 +123,7 @@ use embedder_traits::user_content_manager::UserContentManager;
 use embedder_traits::{
     Cursor, EmbedderMsg, EmbedderProxy, ImeEvent, InputEvent, MediaSessionActionType,
     MediaSessionEvent, MediaSessionPlaybackState, MouseButton, MouseButtonAction, MouseButtonEvent,
-    Theme, WebDriverCommandMsg, WebDriverLoadStatus,
+    Theme, ViewportDetails, WebDriverCommandMsg, WebDriverLoadStatus,
 };
 use euclid::Size2D;
 use euclid::default::Size2D as UntypedSize2D;
@@ -415,9 +415,6 @@ pub struct Constellation<STF, SWF> {
     /// and the namespaces are allocated by the constellation.
     next_pipeline_namespace_id: PipelineNamespaceId,
 
-    /// The size of the top-level window.
-    window_size: WindowSizeData,
-
     /// Bits of state used to interact with the webdriver implementation
     webdriver: WebDriverData,
 
@@ -601,7 +598,6 @@ where
     pub fn start(
         state: InitialConstellationState,
         layout_factory: Arc<dyn LayoutFactory>,
-        initial_window_size: WindowSizeData,
         random_pipeline_closure_probability: Option<f32>,
         random_pipeline_closure_seed: Option<usize>,
         hard_fail: bool,
@@ -713,7 +709,6 @@ where
                     next_pipeline_namespace_id: PipelineNamespaceId(2),
                     time_profiler_chan: state.time_profiler_chan,
                     mem_profiler_chan: state.mem_profiler_chan,
-                    window_size: initial_window_size,
                     phantom: PhantomData,
                     webdriver: WebDriverData::new(),
                     document_states: HashMap::new(),
@@ -877,7 +872,7 @@ where
         webview_id: WebViewId,
         parent_pipeline_id: Option<PipelineId>,
         opener: Option<BrowsingContextId>,
-        initial_window_size: Size2D<f32, CSSPixel>,
+        initial_viewport_details: ViewportDetails,
         // TODO: we have to provide ownership of the LoadData
         // here, because it will be send on an ipc channel,
         // and ipc channels take onership of their data.
@@ -967,10 +962,7 @@ where
             resource_threads,
             time_profiler_chan: self.time_profiler_chan.clone(),
             mem_profiler_chan: self.mem_profiler_chan.clone(),
-            window_size: WindowSizeData {
-                initial_viewport: initial_window_size,
-                device_pixel_ratio: self.window_size.device_pixel_ratio,
-            },
+            viewport_details: initial_viewport_details,
             event_loop,
             load_data,
             prev_throttled: throttled,
@@ -1048,7 +1040,7 @@ where
         top_level_id: WebViewId,
         pipeline_id: PipelineId,
         parent_pipeline_id: Option<PipelineId>,
-        size: Size2D<f32, CSSPixel>,
+        viewport_details: ViewportDetails,
         is_private: bool,
         inherited_secure_context: Option<bool>,
         throttled: bool,
@@ -1081,7 +1073,7 @@ where
             top_level_id,
             pipeline_id,
             parent_pipeline_id,
-            size,
+            viewport_details,
             is_private,
             inherited_secure_context,
             throttled,
@@ -1320,8 +1312,8 @@ where
             },
             // Create a new top level browsing context. Will use response_chan to return
             // the browsing context id.
-            FromCompositorMsg::NewWebView(url, webview_id) => {
-                self.handle_new_top_level_browsing_context(url, webview_id, None);
+            FromCompositorMsg::NewWebView(url, webview_id, viewport_details) => {
+                self.handle_new_top_level_browsing_context(url, webview_id, viewport_details, None);
             },
             // Close a top level browsing context.
             FromCompositorMsg::CloseWebView(webview_id) => {
@@ -1346,8 +1338,16 @@ where
             FromCompositorMsg::TraverseHistory(webview_id, direction) => {
                 self.handle_traverse_history_msg(webview_id, direction);
             },
-            FromCompositorMsg::WindowSize(webview_id, new_size, size_type) => {
-                self.handle_window_size_msg(webview_id, new_size, size_type);
+            FromCompositorMsg::ChangeViewportDetails(
+                webview_id,
+                new_viewport_details,
+                size_type,
+            ) => {
+                self.handle_change_viewport_details_msg(
+                    webview_id,
+                    new_viewport_details,
+                    size_type,
+                );
             },
             FromCompositorMsg::ThemeChange(theme) => {
                 self.handle_theme_change(theme);
@@ -2714,7 +2714,7 @@ where
             Some(context) => context,
             None => return warn!("failed browsing context is missing"),
         };
-        let window_size = browsing_context.size;
+        let viewport_details = browsing_context.viewport_details;
         let pipeline_id = browsing_context.pipeline_id;
         let throttled = browsing_context.throttled;
 
@@ -2759,7 +2759,7 @@ where
             webview_id,
             None,
             opener,
-            window_size,
+            viewport_details,
             new_load_data,
             sandbox,
             is_private,
@@ -2773,7 +2773,7 @@ where
             // to avoid closing again in handle_activate_document_msg (though it would be harmless)
             replace: Some(NeedsToReload::Yes(old_pipeline_id, old_load_data)),
             new_browsing_context_info: None,
-            window_size,
+            viewport_details,
         });
     }
 
@@ -2950,9 +2950,9 @@ where
         &mut self,
         url: ServoUrl,
         webview_id: WebViewId,
+        viewport_details: ViewportDetails,
         response_sender: Option<IpcSender<WebDriverLoadStatus>>,
     ) {
-        let window_size = self.window_size.initial_viewport;
         let pipeline_id = PipelineId::new();
         let browsing_context_id = BrowsingContextId::from(webview_id);
         let load_data = LoadData::new(
@@ -2993,7 +2993,7 @@ where
             webview_id,
             None,
             None,
-            window_size,
+            viewport_details,
             load_data,
             sandbox,
             is_private,
@@ -3010,7 +3010,7 @@ where
                 inherited_secure_context: None,
                 throttled,
             }),
-            window_size,
+            viewport_details,
         });
 
         if let Some(response_sender) = response_sender {
@@ -3067,12 +3067,7 @@ where
             type_,
         } in iframe_sizes
         {
-            let window_size = WindowSizeData {
-                initial_viewport: size,
-                device_pixel_ratio: self.window_size.device_pixel_ratio,
-            };
-
-            self.resize_browsing_context(window_size, type_, browsing_context_id);
+            self.resize_browsing_context(size, type_, browsing_context_id);
         }
     }
 
@@ -3200,14 +3195,13 @@ where
             None
         };
 
-        // https://github.com/rust-lang/rust/issues/59159
-        let browsing_context_size = browsing_context.size;
+        let browsing_context_size = browsing_context.viewport_details;
         let browsing_context_throttled = browsing_context.throttled;
         // TODO(servo#30571) revert to debug_assert_eq!() once underlying bug is fixed
         #[cfg(debug_assertions)]
-        if !(browsing_context_size == load_info.window_size.initial_viewport) {
+        if !(browsing_context_size == load_info.viewport_details) {
             log::warn!(
-                "debug assertion failed! browsing_context_size == load_info.window_size.initial_viewport"
+                "debug assertion failed! browsing_context_size == load_info.viewport_details.initial_viewport"
             );
         }
 
@@ -3231,7 +3225,7 @@ where
             replace,
             // Browsing context for iframe already exists.
             new_browsing_context_info: None,
-            window_size: load_info.window_size.initial_viewport,
+            viewport_details: load_info.viewport_details,
         });
     }
 
@@ -3295,7 +3289,7 @@ where
                 inherited_secure_context: is_parent_secure,
                 throttled: is_parent_throttled,
             }),
-            window_size: load_info.window_size.initial_viewport,
+            viewport_details: load_info.viewport_details,
         });
     }
 
@@ -3323,8 +3317,8 @@ where
             opener_webview_id,
             webview_id_sender,
         ));
-        let new_webview_id = match webview_id_receiver.recv() {
-            Ok(Some(webview_id)) => webview_id,
+        let (new_webview_id, viewport_details) = match webview_id_receiver.recv() {
+            Ok(Some((webview_id, viewport_details))) => (webview_id, viewport_details),
             Ok(None) | Err(_) => {
                 let _ = response_sender.send(None);
                 return;
@@ -3409,7 +3403,7 @@ where
                 inherited_secure_context: is_opener_secure,
                 throttled: is_opener_throttled,
             }),
-            window_size: self.window_size.initial_viewport,
+            viewport_details,
         });
     }
 
@@ -3528,10 +3522,10 @@ where
                 return None;
             },
         };
-        let (window_size, pipeline_id, parent_pipeline_id, is_private, is_throttled) =
+        let (viewport_details, pipeline_id, parent_pipeline_id, is_private, is_throttled) =
             match self.browsing_contexts.get(&browsing_context_id) {
                 Some(ctx) => (
-                    ctx.size,
+                    ctx.viewport_details,
                     ctx.pipeline_id,
                     ctx.parent_pipeline_id,
                     ctx.is_private,
@@ -3608,7 +3602,7 @@ where
                     webview_id,
                     None,
                     opener,
-                    window_size,
+                    viewport_details,
                     load_data,
                     sandbox,
                     is_private,
@@ -3621,7 +3615,7 @@ where
                     replace,
                     // `load_url` is always invoked on an existing browsing context.
                     new_browsing_context_info: None,
-                    window_size,
+                    viewport_details,
                 });
                 Some(new_pipeline_id)
             },
@@ -3901,7 +3895,7 @@ where
                     top_level_id,
                     old_pipeline_id,
                     parent_pipeline_id,
-                    window_size,
+                    viewport_details,
                     is_private,
                     throttled,
                 ) = match self.browsing_contexts.get(&browsing_context_id) {
@@ -3909,7 +3903,7 @@ where
                         ctx.top_level_id,
                         ctx.pipeline_id,
                         ctx.parent_pipeline_id,
-                        ctx.size,
+                        ctx.viewport_details,
                         ctx.is_private,
                         ctx.throttled,
                     ),
@@ -3926,7 +3920,7 @@ where
                     top_level_id,
                     parent_pipeline_id,
                     opener,
-                    window_size,
+                    viewport_details,
                     load_data.clone(),
                     sandbox,
                     is_private,
@@ -3939,7 +3933,7 @@ where
                     replace: Some(NeedsToReload::Yes(pipeline_id, load_data)),
                     // Browsing context must exist at this point.
                     new_browsing_context_info: None,
-                    window_size,
+                    viewport_details,
                 });
                 return;
             },
@@ -4348,14 +4342,15 @@ where
                 };
                 self.embedder_proxy
                     .send(EmbedderMsg::AllowOpeningWebView(webview_id, chan));
-                let webview_id = match port.recv() {
-                    Ok(Some(webview_id)) => webview_id,
+                let (webview_id, viewport_details) = match port.recv() {
+                    Ok(Some((webview_id, viewport_details))) => (webview_id, viewport_details),
                     Ok(None) => return warn!("Embedder refused to allow opening webview"),
                     Err(error) => return warn!("Failed to receive webview id: {error:?}"),
                 };
                 self.handle_new_top_level_browsing_context(
                     ServoUrl::parse_with_base(None, "about:blank").expect("Infallible parse"),
                     webview_id,
+                    viewport_details,
                     Some(load_sender),
                 );
                 let _ = sender.send(webview_id);
@@ -4363,8 +4358,14 @@ where
             WebDriverCommandMsg::FocusWebView(webview_id) => {
                 self.handle_focus_web_view(webview_id);
             },
-            WebDriverCommandMsg::GetWindowSize(_, response_sender) => {
-                let _ = response_sender.send(self.window_size.initial_viewport);
+            WebDriverCommandMsg::GetWindowSize(webview_id, response_sender) => {
+                let browsing_context_id = BrowsingContextId::from(webview_id);
+                let size = self
+                    .browsing_contexts
+                    .get(&browsing_context_id)
+                    .map(|browsing_context| browsing_context.viewport_details.size)
+                    .unwrap_or_default();
+                let _ = response_sender.send(size);
             },
             WebDriverCommandMsg::SetWindowSize(webview_id, size, response_sender) => {
                 self.webdriver.resize_channel = Some(response_sender);
@@ -4726,7 +4727,7 @@ where
                     change.webview_id,
                     change.new_pipeline_id,
                     new_context_info.parent_pipeline_id,
-                    change.window_size,
+                    change.viewport_details,
                     new_context_info.is_private,
                     new_context_info.inherited_secure_context,
                     new_context_info.throttled,
@@ -4969,25 +4970,23 @@ where
         feature = "tracing",
         tracing::instrument(skip_all, fields(servo_profiling = true), level = "trace")
     )]
-    fn handle_window_size_msg(
+    fn handle_change_viewport_details_msg(
         &mut self,
         webview_id: WebViewId,
-        new_size: WindowSizeData,
+        new_viewport_details: ViewportDetails,
         size_type: WindowSizeType,
     ) {
         debug!(
-            "handle_window_size_msg: {:?}",
-            new_size.initial_viewport.to_untyped()
+            "handle_change_viewport_details_msg: {:?}",
+            new_viewport_details.size.to_untyped()
         );
 
         let browsing_context_id = BrowsingContextId::from(webview_id);
-        self.resize_browsing_context(new_size, size_type, browsing_context_id);
+        self.resize_browsing_context(new_viewport_details, size_type, browsing_context_id);
 
         if let Some(response_sender) = self.webdriver.resize_channel.take() {
-            let _ = response_sender.send(new_size.initial_viewport);
+            let _ = response_sender.send(new_viewport_details.size);
         }
-
-        self.window_size = new_size;
     }
 
     /// Called when the window exits from fullscreen mode
@@ -5061,7 +5060,7 @@ where
             // If the rectangle for this pipeline is zero sized, it will
             // never be painted. In this case, don't query the layout
             // thread as it won't contribute to the final output image.
-            if browsing_context.size == Size2D::zero() {
+            if browsing_context.viewport_details.size == Size2D::zero() {
                 continue;
             }
 
@@ -5156,12 +5155,12 @@ where
     )]
     fn resize_browsing_context(
         &mut self,
-        new_size: WindowSizeData,
+        new_viewport_details: ViewportDetails,
         size_type: WindowSizeType,
         browsing_context_id: BrowsingContextId,
     ) {
         if let Some(browsing_context) = self.browsing_contexts.get_mut(&browsing_context_id) {
-            browsing_context.size = new_size.initial_viewport;
+            browsing_context.viewport_details = new_viewport_details;
             // Send Resize (or ResizeInactive) messages to each pipeline in the frame tree.
             let pipeline_id = browsing_context.pipeline_id;
             let pipeline = match self.pipelines.get(&pipeline_id) {
@@ -5170,7 +5169,7 @@ where
             };
             let _ = pipeline.event_loop.send(ScriptThreadMessage::Resize(
                 pipeline.id,
-                new_size,
+                new_viewport_details,
                 size_type,
             ));
             let pipeline_ids = browsing_context
@@ -5181,7 +5180,10 @@ where
                 if let Some(pipeline) = self.pipelines.get(id) {
                     let _ = pipeline
                         .event_loop
-                        .send(ScriptThreadMessage::ResizeInactive(pipeline.id, new_size));
+                        .send(ScriptThreadMessage::ResizeInactive(
+                            pipeline.id,
+                            new_viewport_details,
+                        ));
                 }
             }
         }
@@ -5199,7 +5201,7 @@ where
             if pipeline.browsing_context_id == browsing_context_id {
                 let _ = pipeline.event_loop.send(ScriptThreadMessage::Resize(
                     pipeline.id,
-                    new_size,
+                    new_viewport_details,
                     size_type,
                 ));
             }
diff --git a/components/constellation/pipeline.rs b/components/constellation/pipeline.rs
index fb046d77c33..ffbc3a9b772 100644
--- a/components/constellation/pipeline.rs
+++ b/components/constellation/pipeline.rs
@@ -19,9 +19,9 @@ use base::id::{
 use bluetooth_traits::BluetoothRequest;
 use canvas_traits::webgl::WebGLPipeline;
 use compositing_traits::{CompositionPipeline, CompositorMsg, CompositorProxy};
-use constellation_traits::WindowSizeData;
 use crossbeam_channel::{Sender, unbounded};
 use devtools_traits::{DevtoolsControlMsg, ScriptToDevtoolsControlMsg};
+use embedder_traits::ViewportDetails;
 use embedder_traits::user_content_manager::UserContentManager;
 use fonts::{SystemFontServiceProxy, SystemFontServiceProxySender};
 use ipc_channel::Error;
@@ -161,8 +161,8 @@ pub struct InitialPipelineState {
     /// A channel to the memory profiler thread.
     pub mem_profiler_chan: profile_mem::ProfilerChan,
 
-    /// Information about the initial window size.
-    pub window_size: WindowSizeData,
+    /// The initial [`ViewportDetails`] to use when starting this new [`Pipeline`].
+    pub viewport_details: ViewportDetails,
 
     /// The ID of the pipeline namespace for this script thread.
     pub pipeline_namespace_id: PipelineNamespaceId,
@@ -219,7 +219,7 @@ impl Pipeline {
                     webview_id: state.webview_id,
                     opener: state.opener,
                     load_data: state.load_data.clone(),
-                    window_size: state.window_size,
+                    viewport_details: state.viewport_details,
                 };
 
                 if let Err(e) = script_chan.send(ScriptThreadMessage::AttachLayout(new_layout_info))
@@ -275,7 +275,7 @@ impl Pipeline {
                     resource_threads: state.resource_threads,
                     time_profiler_chan: state.time_profiler_chan,
                     mem_profiler_chan: state.mem_profiler_chan,
-                    window_size: state.window_size,
+                    viewport_details: state.viewport_details,
                     script_chan: script_chan.clone(),
                     load_data: state.load_data.clone(),
                     script_port,
@@ -484,7 +484,7 @@ pub struct UnprivilegedPipelineContent {
     resource_threads: ResourceThreads,
     time_profiler_chan: time::ProfilerChan,
     mem_profiler_chan: profile_mem::ProfilerChan,
-    window_size: WindowSizeData,
+    viewport_details: ViewportDetails,
     script_chan: IpcSender<ScriptThreadMessage>,
     load_data: LoadData,
     script_port: IpcReceiver<ScriptThreadMessage>,
@@ -534,7 +534,7 @@ impl UnprivilegedPipelineContent {
                 time_profiler_sender: self.time_profiler_chan.clone(),
                 memory_profiler_sender: self.mem_profiler_chan.clone(),
                 devtools_server_sender: self.devtools_ipc_sender,
-                window_size: self.window_size,
+                viewport_details: self.viewport_details,
                 pipeline_namespace_id: self.pipeline_namespace_id,
                 content_process_shutdown_sender: content_process_shutdown_chan,
                 webgl_chan: self.webgl_chan,
diff --git a/components/constellation/session_history.rs b/components/constellation/session_history.rs
index 9155ffcfca8..1da2ea4db65 100644
--- a/components/constellation/session_history.rs
+++ b/components/constellation/session_history.rs
@@ -6,11 +6,10 @@ use std::cmp::PartialEq;
 use std::fmt;
 
 use base::id::{BrowsingContextId, HistoryStateId, PipelineId, WebViewId};
-use euclid::Size2D;
+use embedder_traits::ViewportDetails;
 use log::debug;
 use script_traits::LoadData;
 use servo_url::ServoUrl;
-use style_traits::CSSPixel;
 
 use crate::browsingcontext::NewBrowsingContextInfo;
 
@@ -127,8 +126,8 @@ pub struct SessionHistoryChange {
     /// easily available when they need to be constructed.
     pub new_browsing_context_info: Option<NewBrowsingContextInfo>,
 
-    /// The size of the viewport for the browsing context.
-    pub window_size: Size2D<f32, CSSPixel>,
+    /// The size and hidpi scale factor of the viewport for the browsing context.
+    pub viewport_details: ViewportDetails,
 }
 
 /// Represents a pipeline or discarded pipeline in a history entry.
diff --git a/components/constellation/tracing.rs b/components/constellation/tracing.rs
index fd00d62067b..d7373ef4d93 100644
--- a/components/constellation/tracing.rs
+++ b/components/constellation/tracing.rs
@@ -58,7 +58,7 @@ mod from_compositor {
                 Self::LoadUrl(..) => target!("LoadUrl"),
                 Self::ClearCache => target!("ClearCache"),
                 Self::TraverseHistory(..) => target!("TraverseHistory"),
-                Self::WindowSize(..) => target!("WindowSize"),
+                Self::ChangeViewportDetails(..) => target!("ChangeViewportDetails"),
                 Self::ThemeChange(..) => target!("ThemeChange"),
                 Self::TickAnimation(..) => target!("TickAnimation"),
                 Self::WebDriverCommand(..) => target!("WebDriverCommand"),
diff --git a/components/layout_2020/replaced.rs b/components/layout_2020/replaced.rs
index 47bb932fa9e..58d09700e6e 100644
--- a/components/layout_2020/replaced.rs
+++ b/components/layout_2020/replaced.rs
@@ -9,7 +9,8 @@ use std::sync::Arc;
 use app_units::Au;
 use base::id::{BrowsingContextId, PipelineId};
 use data_url::DataUrl;
-use euclid::Size2D;
+use embedder_traits::ViewportDetails;
+use euclid::{Scale, Size2D};
 use net_traits::image_cache::{ImageOrMetadataAvailable, UsePlaceholder};
 use pixels::Image;
 use script_layout_interface::IFrameSize;
@@ -358,12 +359,17 @@ impl ReplacedContents {
             },
             ReplacedContentKind::IFrame(iframe) => {
                 let size = Size2D::new(rect.size.width.to_f32_px(), rect.size.height.to_f32_px());
+                let hidpi_scale_factor = layout_context.shared_context().device_pixel_ratio();
+
                 layout_context.iframe_sizes.lock().insert(
                     iframe.browsing_context_id,
                     IFrameSize {
                         browsing_context_id: iframe.browsing_context_id,
                         pipeline_id: iframe.pipeline_id,
-                        size,
+                        viewport_details: ViewportDetails {
+                            size,
+                            hidpi_scale_factor: Scale::new(hidpi_scale_factor.0),
+                        },
                     },
                 );
                 vec![Fragment::IFrame(ArcRefCell::new(IFrameFragment {
diff --git a/components/layout_thread_2020/lib.rs b/components/layout_thread_2020/lib.rs
index 470ce1d7600..a34a7bf75a5 100644
--- a/components/layout_thread_2020/lib.rs
+++ b/components/layout_thread_2020/lib.rs
@@ -17,7 +17,8 @@ use std::sync::{Arc, LazyLock};
 use app_units::Au;
 use base::Epoch;
 use base::id::{PipelineId, WebViewId};
-use constellation_traits::{ScrollState, UntrustedNodeAddress, WindowSizeData};
+use constellation_traits::{ScrollState, UntrustedNodeAddress};
+use embedder_traits::ViewportDetails;
 use embedder_traits::resources::{self, Resource};
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect, Size2D as UntypedSize2D};
 use euclid::{Point2D, Scale, Size2D, Vector2D};
@@ -473,8 +474,8 @@ impl LayoutThread {
         let device = Device::new(
             MediaType::screen(),
             QuirksMode::NoQuirks,
-            config.window_size.initial_viewport,
-            Scale::new(config.window_size.device_pixel_ratio.get()),
+            config.viewport_details.size,
+            Scale::new(config.viewport_details.hidpi_scale_factor.get()),
             Box::new(LayoutFontMetricsProvider(config.font_context.clone())),
             ComputedValues::initial_values_with_font_override(font),
             // TODO: obtain preferred color scheme from embedder
@@ -497,8 +498,8 @@ impl LayoutThread {
             // Epoch starts at 1 because of the initial display list for epoch 0 that we send to WR
             epoch: Cell::new(Epoch(1)),
             viewport_size: Size2D::new(
-                Au::from_f32_px(config.window_size.initial_viewport.width),
-                Au::from_f32_px(config.window_size.initial_viewport.height),
+                Au::from_f32_px(config.viewport_details.size.width),
+                Au::from_f32_px(config.viewport_details.size.height),
             ),
             compositor_api: config.compositor_api,
             scroll_offsets: Default::default(),
@@ -618,11 +619,15 @@ impl LayoutThread {
         };
 
         let had_used_viewport_units = self.stylist.device().used_viewport_units();
-        let viewport_size_changed = self.viewport_did_change(reflow_request.window_size);
+        let viewport_size_changed = self.viewport_did_change(reflow_request.viewport_details);
         let theme_changed = self.theme_did_change(reflow_request.theme);
 
         if viewport_size_changed || theme_changed {
-            self.update_device(reflow_request.window_size, reflow_request.theme, &guards);
+            self.update_device(
+                reflow_request.viewport_details,
+                reflow_request.theme,
+                &guards,
+            );
         }
 
         if viewport_size_changed && had_used_viewport_units {
@@ -951,11 +956,11 @@ impl LayoutThread {
         }
     }
 
-    fn viewport_did_change(&mut self, window_size_data: WindowSizeData) -> bool {
-        let new_pixel_ratio = window_size_data.device_pixel_ratio.get();
+    fn viewport_did_change(&mut self, viewport_details: ViewportDetails) -> bool {
+        let new_pixel_ratio = viewport_details.hidpi_scale_factor.get();
         let new_viewport_size = Size2D::new(
-            Au::from_f32_px(window_size_data.initial_viewport.width),
-            Au::from_f32_px(window_size_data.initial_viewport.height),
+            Au::from_f32_px(viewport_details.size.width),
+            Au::from_f32_px(viewport_details.size.height),
         );
 
         // TODO: eliminate self.viewport_size in favour of using self.device.au_viewport_size()
@@ -975,15 +980,15 @@ impl LayoutThread {
     /// Update layout given a new viewport. Returns true if the viewport changed or false if it didn't.
     fn update_device(
         &mut self,
-        window_size_data: WindowSizeData,
+        viewport_details: ViewportDetails,
         theme: PrefersColorScheme,
         guards: &StylesheetGuards,
     ) {
         let device = Device::new(
             MediaType::screen(),
             self.stylist.quirks_mode(),
-            window_size_data.initial_viewport,
-            Scale::new(window_size_data.device_pixel_ratio.get()),
+            viewport_details.size,
+            Scale::new(viewport_details.hidpi_scale_factor.get()),
             Box::new(LayoutFontMetricsProvider(self.font_context.clone())),
             self.stylist.device().default_computed_values().to_arc(),
             theme,
diff --git a/components/script/dom/documentorshadowroot.rs b/components/script/dom/documentorshadowroot.rs
index ac9140e65c4..6d59623c966 100644
--- a/components/script/dom/documentorshadowroot.rs
+++ b/components/script/dom/documentorshadowroot.rs
@@ -128,7 +128,7 @@ impl DocumentOrShadowRoot {
         let x = *x as f32;
         let y = *y as f32;
         let point = &Point2D::new(x, y);
-        let viewport = self.window.window_size().initial_viewport;
+        let viewport = self.window.viewport_details().size;
 
         if !has_browsing_context {
             return None;
@@ -176,7 +176,7 @@ impl DocumentOrShadowRoot {
         let x = *x as f32;
         let y = *y as f32;
         let point = &Point2D::new(x, y);
-        let viewport = self.window.window_size().initial_viewport;
+        let viewport = self.window.viewport_details().size;
 
         if !has_browsing_context {
             return vec![];
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index db0b4c63c5f..8193c99c896 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -4344,7 +4344,7 @@ impl Element {
         if (in_quirks_mode && doc.GetBody().as_deref() == self.downcast::<HTMLElement>()) ||
             (!in_quirks_mode && *self.root_element() == *self)
         {
-            let viewport_dimensions = doc.window().window_size().initial_viewport.round().to_i32();
+            let viewport_dimensions = doc.window().viewport_details().size.round().to_i32();
             rect.size = Size2D::<i32>::new(viewport_dimensions.width, viewport_dimensions.height);
         }
 
diff --git a/components/script/dom/htmliframeelement.rs b/components/script/dom/htmliframeelement.rs
index dd4277d18cb..9a680353ad9 100644
--- a/components/script/dom/htmliframeelement.rs
+++ b/components/script/dom/htmliframeelement.rs
@@ -6,8 +6,8 @@ use std::cell::Cell;
 
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
 use bitflags::bitflags;
-use constellation_traits::WindowSizeData;
 use dom_struct::dom_struct;
+use embedder_traits::ViewportDetails;
 use html5ever::{LocalName, Prefix, local_name, namespace_url, ns};
 use js::rust::HandleObject;
 use net_traits::ReferrerPolicy;
@@ -196,12 +196,12 @@ impl HTMLIFrameElement {
             history_handling,
         };
 
-        let window_size = WindowSizeData {
-            initial_viewport: window
-                .get_iframe_size_if_known(browsing_context_id, can_gc)
-                .unwrap_or_default(),
-            device_pixel_ratio: window.device_pixel_ratio(),
-        };
+        let viewport_details = window
+            .get_iframe_viewport_details_if_known(browsing_context_id, can_gc)
+            .unwrap_or_else(|| ViewportDetails {
+                hidpi_scale_factor: window.device_pixel_ratio(),
+                ..Default::default()
+            });
 
         match pipeline_type {
             PipelineType::InitialAboutBlank => {
@@ -212,7 +212,7 @@ impl HTMLIFrameElement {
                     load_data: load_data.clone(),
                     old_pipeline_id,
                     sandbox: sandboxed,
-                    window_size,
+                    viewport_details,
                 };
                 window
                     .as_global_scope()
@@ -227,7 +227,7 @@ impl HTMLIFrameElement {
                     webview_id,
                     opener: None,
                     load_data,
-                    window_size,
+                    viewport_details,
                 };
 
                 self.pipeline_id.set(Some(new_pipeline_id));
@@ -239,7 +239,7 @@ impl HTMLIFrameElement {
                     load_data,
                     old_pipeline_id,
                     sandbox: sandboxed,
-                    window_size,
+                    viewport_details,
                 };
                 window
                     .as_global_scope()
diff --git a/components/script/dom/htmlimageelement.rs b/components/script/dom/htmlimageelement.rs
index bec3b2018f7..ee23270a549 100644
--- a/components/script/dom/htmlimageelement.rs
+++ b/components/script/dom/htmlimageelement.rs
@@ -808,8 +808,8 @@ impl HTMLImageElement {
         let device_pixel_ratio = self
             .owner_document()
             .window()
-            .window_size()
-            .device_pixel_ratio
+            .viewport_details()
+            .hidpi_scale_factor
             .get() as f64;
         for (index, image_source) in img_sources.iter().enumerate() {
             let current_den = image_source.descriptor.density.unwrap();
diff --git a/components/script/dom/intersectionobserver.rs b/components/script/dom/intersectionobserver.rs
index 81ac8a1337d..ec98116d3a4 100644
--- a/components/script/dom/intersectionobserver.rs
+++ b/components/script/dom/intersectionobserver.rs
@@ -444,7 +444,7 @@ impl IntersectionObserver {
                 // > (note that this processing step can only be reached if the document is fully active).
                 // TODO: viewport should consider native scrollbar if exist. Recheck Servo's scrollbar approach.
                 document.map(|document| {
-                    let viewport = document.window().window_size().initial_viewport;
+                    let viewport = document.window().viewport_details().size;
                     Rect::from_size(Size2D::new(
                         Au::from_f32_px(viewport.width),
                         Au::from_f32_px(viewport.height),
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 84f8deb16b1..57c90354dba 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -21,7 +21,7 @@ use base64::Engine;
 #[cfg(feature = "bluetooth")]
 use bluetooth_traits::BluetoothRequest;
 use canvas_traits::webgl::WebGLChan;
-use constellation_traits::{ScrollState, WindowSizeData, WindowSizeType};
+use constellation_traits::{ScrollState, WindowSizeType};
 use crossbeam_channel::{Sender, unbounded};
 use cssparser::{Parser, ParserInput, SourceLocation};
 use devtools_traits::{ScriptToDevtoolsControlMsg, TimelineMarker, TimelineMarkerType};
@@ -29,7 +29,7 @@ use dom_struct::dom_struct;
 use embedder_traits::user_content_manager::{UserContentManager, UserScript};
 use embedder_traits::{
     AlertResponse, ConfirmResponse, EmbedderMsg, PromptResponse, SimpleDialog, Theme,
-    WebDriverJSError, WebDriverJSResult,
+    ViewportDetails, WebDriverJSError, WebDriverJSResult,
 };
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect};
 use euclid::{Point2D, Rect, Scale, Size2D, Vector2D};
@@ -257,7 +257,7 @@ pub(crate) struct Window {
 
     /// Most recent unhandled resize event, if any.
     #[no_trace]
-    unhandled_resize_event: DomRefCell<Option<(WindowSizeData, WindowSizeType)>>,
+    unhandled_resize_event: DomRefCell<Option<(ViewportDetails, WindowSizeType)>>,
 
     /// Platform theme.
     #[no_trace]
@@ -274,9 +274,9 @@ pub(crate) struct Window {
     #[ignore_malloc_size_of = "Rc<T> is hard"]
     js_runtime: DomRefCell<Option<Rc<Runtime>>>,
 
-    /// The current size of the window, in pixels.
+    /// The [`ViewportDetails`] of this [`Window`]'s frame.
     #[no_trace]
-    window_size: Cell<WindowSizeData>,
+    viewport_details: Cell<ViewportDetails>,
 
     /// A handle for communicating messages to the bluetooth thread.
     #[no_trace]
@@ -1300,9 +1300,9 @@ impl WindowMethods<crate::DomTypeHolder> for Window {
     // https://drafts.csswg.org/cssom-view/#dom-window-innerheight
     //TODO Include Scrollbar
     fn InnerHeight(&self) -> i32 {
-        self.window_size
+        self.viewport_details
             .get()
-            .initial_viewport
+            .size
             .height
             .to_i32()
             .unwrap_or(0)
@@ -1311,12 +1311,7 @@ impl WindowMethods<crate::DomTypeHolder> for Window {
     // https://drafts.csswg.org/cssom-view/#dom-window-innerwidth
     //TODO Include Scrollbar
     fn InnerWidth(&self) -> i32 {
-        self.window_size
-            .get()
-            .initial_viewport
-            .width
-            .to_i32()
-            .unwrap_or(0)
+        self.viewport_details.get().size.width.to_i32().unwrap_or(0)
     }
 
     // https://drafts.csswg.org/cssom-view/#dom-window-scrollx
@@ -1822,7 +1817,7 @@ impl Window {
 
         // Step 5 & 6
         // TODO: Remove scrollbar dimensions.
-        let viewport = self.window_size.get().initial_viewport;
+        let viewport = self.viewport_details.get().size;
 
         // Step 7 & 8
         // TODO: Consider `block-end` and `inline-end` overflow direction.
@@ -1885,7 +1880,7 @@ impl Window {
     }
 
     pub(crate) fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel> {
-        self.window_size.get().device_pixel_ratio
+        self.viewport_details.get().hidpi_scale_factor
     }
 
     fn client_window(&self) -> (Size2D<u32, CSSPixel>, Point2D<i32, CSSPixel>) {
@@ -1994,7 +1989,7 @@ impl Window {
             document: document.upcast::<Node>().to_trusted_node_address(),
             dirty_root,
             stylesheets_changed,
-            window_size: self.window_size.get(),
+            viewport_details: self.viewport_details.get(),
             origin: self.origin().immutable().clone(),
             reflow_goal,
             dom_count: document.dom_count(),
@@ -2050,7 +2045,7 @@ impl Window {
         let size_messages = self
             .Document()
             .iframes_mut()
-            .handle_new_iframe_sizes_after_layout(results.iframe_sizes, self.device_pixel_ratio());
+            .handle_new_iframe_sizes_after_layout(results.iframe_sizes);
         if !size_messages.is_empty() {
             self.send_to_constellation(ScriptMsg::IFrameSizes(size_messages));
         }
@@ -2364,11 +2359,11 @@ impl Window {
     /// If the given |browsing_context_id| refers to an `<iframe>` that is an element
     /// in this [`Window`] and that `<iframe>` has been laid out, return its size.
     /// Otherwise, return `None`.
-    pub(crate) fn get_iframe_size_if_known(
+    pub(crate) fn get_iframe_viewport_details_if_known(
         &self,
         browsing_context_id: BrowsingContextId,
         can_gc: CanGc,
-    ) -> Option<Size2D<f32, CSSPixel>> {
+    ) -> Option<ViewportDetails> {
         // Reflow might fail, but do a best effort to return the right size.
         self.layout_reflow(QueryMsg::InnerWindowDimensionsQuery, can_gc);
         self.Document()
@@ -2533,12 +2528,12 @@ impl Window {
         };
     }
 
-    pub(crate) fn set_window_size(&self, size: WindowSizeData) {
-        self.window_size.set(size);
+    pub(crate) fn set_viewport_details(&self, size: ViewportDetails) {
+        self.viewport_details.set(size);
     }
 
-    pub(crate) fn window_size(&self) -> WindowSizeData {
-        self.window_size.get()
+    pub(crate) fn viewport_details(&self) -> ViewportDetails {
+        self.viewport_details.get()
     }
 
     /// Handle a theme change request, triggering a reflow is any actual change occured.
@@ -2563,13 +2558,13 @@ impl Window {
         self.dom_static.windowproxy_handler
     }
 
-    pub(crate) fn add_resize_event(&self, event: WindowSizeData, event_type: WindowSizeType) {
+    pub(crate) fn add_resize_event(&self, event: ViewportDetails, event_type: WindowSizeType) {
         // Whenever we receive a new resize event we forget about all the ones that came before
         // it, to avoid unnecessary relayouts
         *self.unhandled_resize_event.borrow_mut() = Some((event, event_type))
     }
 
-    pub(crate) fn take_unhandled_resize_event(&self) -> Option<(WindowSizeData, WindowSizeType)> {
+    pub(crate) fn take_unhandled_resize_event(&self) -> Option<(ViewportDetails, WindowSizeType)> {
         self.unhandled_resize_event.borrow_mut().take()
     }
 
@@ -2685,7 +2680,7 @@ impl Window {
             return false;
         };
 
-        if self.window_size() == new_size {
+        if self.viewport_details() == new_size {
             return false;
         }
 
@@ -2693,9 +2688,9 @@ impl Window {
         debug!(
             "Resizing Window for pipeline {:?} from {:?} to {new_size:?}",
             self.pipeline_id(),
-            self.window_size(),
+            self.viewport_details(),
         );
-        self.set_window_size(new_size);
+        self.set_viewport_details(new_size);
 
         // http://dev.w3.org/csswg/cssom-view/#resizing-viewports
         if size_type == WindowSizeType::Resize {
@@ -2823,7 +2818,7 @@ impl Window {
         control_chan: IpcSender<ScriptThreadMessage>,
         pipeline_id: PipelineId,
         parent_info: Option<PipelineId>,
-        window_size: WindowSizeData,
+        viewport_details: ViewportDetails,
         origin: MutableOrigin,
         creator_url: ServoUrl,
         navigation_start: CrossProcessInstant,
@@ -2848,7 +2843,7 @@ impl Window {
 
         let initial_viewport = f32_rect_to_au_rect(UntypedRect::new(
             Point2D::zero(),
-            window_size.initial_viewport.to_untyped(),
+            viewport_details.size.to_untyped(),
         ));
 
         let win = Box::new(Self {
@@ -2894,7 +2889,7 @@ impl Window {
             bluetooth_extra_permission_data: BluetoothExtraPermissionData::new(),
             page_clip_rect: Cell::new(MaxRect::max_rect()),
             unhandled_resize_event: Default::default(),
-            window_size: Cell::new(window_size),
+            viewport_details: Cell::new(viewport_details),
             current_viewport: Cell::new(initial_viewport.to_untyped()),
             layout_blocker: Cell::new(LayoutBlocker::WaitingForParse),
             current_state: Cell::new(WindowState::Alive),
diff --git a/components/script/dom/windowproxy.rs b/components/script/dom/windowproxy.rs
index 1278a0af115..4bbdf271252 100644
--- a/components/script/dom/windowproxy.rs
+++ b/components/script/dom/windowproxy.rs
@@ -326,7 +326,7 @@ impl WindowProxy {
             webview_id: response.new_webview_id,
             opener: Some(self.browsing_context_id),
             load_data,
-            window_size: window.window_size(),
+            viewport_details: window.viewport_details(),
         };
         ScriptThread::process_attach_layout(new_layout_info, document.origin().clone());
         // TODO: if noopener is false, copy the sessionStorage storage area of the creator origin.
diff --git a/components/script/iframe_collection.rs b/components/script/iframe_collection.rs
index caf40009fe3..01405881fa6 100644
--- a/components/script/iframe_collection.rs
+++ b/components/script/iframe_collection.rs
@@ -6,13 +6,11 @@ use std::cell::Cell;
 use std::default::Default;
 
 use base::id::BrowsingContextId;
-use constellation_traits::{WindowSizeData, WindowSizeType};
-use euclid::{Scale, Size2D};
+use constellation_traits::WindowSizeType;
+use embedder_traits::ViewportDetails;
 use fnv::FnvHashMap;
 use script_layout_interface::IFrameSizes;
 use script_traits::IFrameSizeMsg;
-use style_traits::CSSPixel;
-use webrender_api::units::DevicePixel;
 
 use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::root::{Dom, DomRoot};
@@ -26,7 +24,7 @@ use crate::script_thread::with_script_thread;
 pub(crate) struct IFrame {
     pub(crate) element: Dom<HTMLIFrameElement>,
     #[no_trace]
-    pub(crate) size: Option<Size2D<f32, CSSPixel>>,
+    pub(crate) size: Option<ViewportDetails>,
 }
 
 #[derive(Default, JSTraceable, MallocSizeOf)]
@@ -98,11 +96,11 @@ impl IFrameCollection {
 
     /// Set the size of an `<iframe>` in the collection given its `BrowsingContextId` and
     /// the new size. Returns the old size.
-    pub(crate) fn set_size(
+    pub(crate) fn set_viewport_details(
         &mut self,
         browsing_context_id: BrowsingContextId,
-        new_size: Size2D<f32, CSSPixel>,
-    ) -> Option<Size2D<f32, CSSPixel>> {
+        new_size: ViewportDetails,
+    ) -> Option<ViewportDetails> {
         self.get_mut(browsing_context_id)
             .expect("Tried to set a size for an unknown <iframe>")
             .size
@@ -115,7 +113,6 @@ impl IFrameCollection {
     pub(crate) fn handle_new_iframe_sizes_after_layout(
         &mut self,
         new_iframe_sizes: IFrameSizes,
-        device_pixel_ratio: Scale<f32, CSSPixel, DevicePixel>,
     ) -> Vec<IFrameSizeMsg> {
         if new_iframe_sizes.is_empty() {
             return vec![];
@@ -123,37 +120,35 @@ impl IFrameCollection {
 
         new_iframe_sizes
             .into_iter()
-            .filter_map(|(browsing_context_id, size)| {
+            .filter_map(|(browsing_context_id, iframe_size)| {
                 // Batch resize message to any local `Pipeline`s now, rather than waiting for them
                 // to filter asynchronously through the `Constellation`. This allows the new value
                 // to be reflected immediately in layout.
-                let new_size = size.size;
+                let viewport_details = iframe_size.viewport_details;
                 with_script_thread(|script_thread| {
                     script_thread.handle_resize_message(
-                        size.pipeline_id,
-                        WindowSizeData {
-                            initial_viewport: new_size,
-                            device_pixel_ratio,
-                        },
+                        iframe_size.pipeline_id,
+                        viewport_details,
                         WindowSizeType::Resize,
                     );
                 });
 
-                let old_size = self.set_size(browsing_context_id, new_size);
+                let old_viewport_details =
+                    self.set_viewport_details(browsing_context_id, viewport_details);
                 // The `Constellation` should be up-to-date even when the in-ScriptThread pipelines
                 // might not be.
-                if old_size == Some(size.size) {
+                if old_viewport_details == Some(viewport_details) {
                     return None;
                 }
 
-                let size_type = match old_size {
+                let size_type = match old_viewport_details {
                     Some(_) => WindowSizeType::Resize,
                     None => WindowSizeType::Initial,
                 };
 
                 Some(IFrameSizeMsg {
                     browsing_context_id,
-                    size: new_size,
+                    size: viewport_details,
                     type_: size_type,
                 })
             })
diff --git a/components/script/navigation.rs b/components/script/navigation.rs
index 761492b79bf..2c22e25c123 100644
--- a/components/script/navigation.rs
+++ b/components/script/navigation.rs
@@ -10,9 +10,9 @@ use std::cell::Cell;
 
 use base::cross_process_instant::CrossProcessInstant;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
-use constellation_traits::WindowSizeData;
 use content_security_policy::Destination;
 use crossbeam_channel::Sender;
+use embedder_traits::ViewportDetails;
 use http::header;
 use net_traits::request::{
     CredentialsMode, InsecureRequestsPolicy, RedirectMode, RequestBuilder, RequestMode,
@@ -138,7 +138,7 @@ pub(crate) struct InProgressLoad {
     pub(crate) opener: Option<BrowsingContextId>,
     /// The current window size associated with this pipeline.
     #[no_trace]
-    pub(crate) window_size: WindowSizeData,
+    pub(crate) viewport_details: ViewportDetails,
     /// The activity level of the document (inactive, active or fully active).
     #[no_trace]
     pub(crate) activity: DocumentActivity,
@@ -170,7 +170,7 @@ impl InProgressLoad {
         webview_id: WebViewId,
         parent_info: Option<PipelineId>,
         opener: Option<BrowsingContextId>,
-        window_size: WindowSizeData,
+        viewport_details: ViewportDetails,
         origin: MutableOrigin,
         load_data: LoadData,
     ) -> InProgressLoad {
@@ -181,7 +181,7 @@ impl InProgressLoad {
             webview_id,
             parent_info,
             opener,
-            window_size,
+            viewport_details,
             activity: DocumentActivity::FullyActive,
             throttled: false,
             origin,
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 42241ae83ab..b3a8762e7ac 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -36,7 +36,7 @@ use base::cross_process_instant::CrossProcessInstant;
 use base::id::{BrowsingContextId, HistoryStateId, PipelineId, PipelineNamespace, WebViewId};
 use canvas_traits::webgl::WebGLPipeline;
 use chrono::{DateTime, Local};
-use constellation_traits::{CompositorHitTestResult, ScrollState, WindowSizeData, WindowSizeType};
+use constellation_traits::{CompositorHitTestResult, ScrollState, WindowSizeType};
 use crossbeam_channel::unbounded;
 use devtools_traits::{
     CSSError, DevtoolScriptControlMsg, DevtoolsPageInfo, NavigationState,
@@ -44,7 +44,7 @@ use devtools_traits::{
 };
 use embedder_traits::user_content_manager::UserContentManager;
 use embedder_traits::{
-    EmbedderMsg, InputEvent, MediaSessionActionType, Theme, WebDriverScriptCommand,
+    EmbedderMsg, InputEvent, MediaSessionActionType, Theme, ViewportDetails, WebDriverScriptCommand,
 };
 use euclid::default::Rect;
 use fonts::{FontContext, SystemFontServiceProxy};
@@ -401,7 +401,7 @@ impl ScriptThreadFactory for ScriptThread {
                 let parent_info = state.parent_info;
                 let opener = state.opener;
                 let memory_profiler_sender = state.memory_profiler_sender.clone();
-                let window_size = state.window_size;
+                let viewport_details = state.viewport_details;
 
                 let script_thread = ScriptThread::new(state, layout_factory, system_font_service);
 
@@ -418,7 +418,7 @@ impl ScriptThreadFactory for ScriptThread {
                     webview_id,
                     parent_info,
                     opener,
-                    window_size,
+                    viewport_details,
                     origin,
                     load_data,
                 ));
@@ -2337,18 +2337,18 @@ impl ScriptThread {
     pub(crate) fn handle_resize_message(
         &self,
         id: PipelineId,
-        size: WindowSizeData,
+        viewport_details: ViewportDetails,
         size_type: WindowSizeType,
     ) {
         self.profile_event(ScriptThreadEventCategory::Resize, Some(id), || {
             let window = self.documents.borrow().find_window(id);
             if let Some(ref window) = window {
-                window.add_resize_event(size, size_type);
+                window.add_resize_event(viewport_details, size_type);
                 return;
             }
             let mut loads = self.incomplete_loads.borrow_mut();
             if let Some(ref mut load) = loads.iter_mut().find(|load| load.pipeline_id == id) {
-                load.window_size = size;
+                load.viewport_details = viewport_details;
             }
         })
     }
@@ -2390,7 +2390,7 @@ impl ScriptThread {
             webview_id,
             opener,
             load_data,
-            window_size,
+            viewport_details,
         } = new_layout_info;
 
         // Kick off the fetch for the new resource.
@@ -2401,7 +2401,7 @@ impl ScriptThread {
             webview_id,
             parent_info,
             opener,
-            window_size,
+            viewport_details,
             origin,
             load_data,
         );
@@ -2647,10 +2647,10 @@ impl ScriptThread {
     }
 
     /// Window was resized, but this script was not active, so don't reflow yet
-    fn handle_resize_inactive_msg(&self, id: PipelineId, new_size: WindowSizeData) {
+    fn handle_resize_inactive_msg(&self, id: PipelineId, new_viewport_details: ViewportDetails) {
         let window = self.documents.borrow().find_window(id)
             .expect("ScriptThread: received a resize msg for a pipeline not in this script thread. This is a bug.");
-        window.set_window_size(new_size);
+        window.set_viewport_details(new_viewport_details);
     }
 
     /// We have received notification that the response associated with a load has completed.
@@ -3066,7 +3066,7 @@ impl ScriptThread {
             font_context: font_context.clone(),
             time_profiler_chan: self.senders.time_profiler_sender.clone(),
             compositor_api: self.compositor_api.clone(),
-            window_size: incomplete.window_size,
+            viewport_details: incomplete.viewport_details,
         };
 
         // Create the window and document objects.
@@ -3088,7 +3088,7 @@ impl ScriptThread {
             self.senders.constellation_sender.clone(),
             incomplete.pipeline_id,
             incomplete.parent_info,
-            incomplete.window_size,
+            incomplete.viewport_details,
             origin.clone(),
             final_url.clone(),
             incomplete.navigation_start,
diff --git a/components/servo/lib.rs b/components/servo/lib.rs
index 624110cc0ad..3bd0e1443a8 100644
--- a/components/servo/lib.rs
+++ b/components/servo/lib.rs
@@ -58,12 +58,11 @@ use constellation::{
     Constellation, FromCompositorLogger, FromScriptLogger, InitialConstellationState,
     UnprivilegedContent,
 };
-use constellation_traits::{ConstellationMsg, WindowSizeData};
+use constellation_traits::ConstellationMsg;
 use crossbeam_channel::{Receiver, Sender, unbounded};
 use embedder_traits::user_content_manager::UserContentManager;
 pub use embedder_traits::*;
 use env_logger::Builder as EnvLoggerBuilder;
-use euclid::Scale;
 use fonts::SystemFontService;
 #[cfg(all(
     not(target_os = "windows"),
@@ -321,9 +320,6 @@ impl Servo {
             None
         };
 
-        let device_pixel_ratio = window.hidpi_factor().get();
-        let viewport_size = rendering_context.size2d();
-
         let (mut webrender, webrender_api_sender) = {
             let mut debug_flags = webrender::DebugFlags::empty();
             debug_flags.set(
@@ -389,7 +385,7 @@ impl Servo {
         };
 
         let webrender_api = webrender_api_sender.create_api();
-        let webrender_document = webrender_api.add_document(viewport_size.to_i32());
+        let webrender_document = webrender_api.add_document(rendering_context.size2d().to_i32());
 
         // Important that this call is done in a single-threaded fashion, we
         // can't defer it after `create_constellation` has started.
@@ -451,14 +447,6 @@ impl Servo {
 
         webrender.set_external_image_handler(external_image_handlers);
 
-        // The division by 1 represents the page's default zoom of 100%,
-        // and gives us the appropriate CSSPixel type for the viewport.
-        let scaled_viewport_size = viewport_size.to_f32().to_untyped() / device_pixel_ratio;
-        let window_size = WindowSizeData {
-            initial_viewport: scaled_viewport_size / Scale::new(1.0),
-            device_pixel_ratio: Scale::new(device_pixel_ratio),
-        };
-
         // Create the constellation, which maintains the engine pipelines, including script and
         // layout, as well as the navigation context.
         let mut protocols = ProtocolRegistry::with_internal_protocols();
@@ -476,7 +464,6 @@ impl Servo {
             #[cfg(feature = "webxr")]
             webxr_main_thread.registry(),
             Some(webgl_threads),
-            window_size,
             external_images,
             #[cfg(feature = "webgpu")]
             wgpu_image_map,
@@ -654,8 +641,12 @@ impl Servo {
         self.webviews
             .borrow_mut()
             .insert(webview.id(), webview.weak_handle());
-        self.constellation_proxy
-            .send(ConstellationMsg::NewWebView(url.into(), webview.id()));
+        let viewport_details = self.compositor.borrow().default_webview_viewport_details();
+        self.constellation_proxy.send(ConstellationMsg::NewWebView(
+            url.into(),
+            webview.id(),
+            viewport_details,
+        ));
         webview
     }
 
@@ -724,8 +715,19 @@ impl Servo {
             },
             EmbedderMsg::AllowOpeningWebView(webview_id, response_sender) => {
                 if let Some(webview) = self.get_webview_handle(webview_id) {
-                    let new_webview = webview.delegate().request_open_auxiliary_webview(webview);
-                    let _ = response_sender.send(new_webview.map(|webview| webview.id()));
+                    let webview_id_and_viewport_details = webview
+                        .delegate()
+                        .request_open_auxiliary_webview(webview)
+                        .map(|webview| {
+                            let mut viewport =
+                                self.compositor.borrow().default_webview_viewport_details();
+                            let rect = webview.rect();
+                            if !rect.is_empty() {
+                                viewport.size = rect.size() / viewport.hidpi_scale_factor;
+                            }
+                            (webview.id(), viewport)
+                        });
+                    let _ = response_sender.send(webview_id_and_viewport_details);
                 }
             },
             EmbedderMsg::WebViewClosed(webview_id) => {
@@ -1042,7 +1044,6 @@ fn create_constellation(
     webrender_api_sender: RenderApiSender,
     #[cfg(feature = "webxr")] webxr_registry: webxr_api::Registry,
     webgl_threads: Option<WebGLThreads>,
-    initial_window_size: WindowSizeData,
     external_images: Arc<Mutex<WebrenderExternalImageRegistry>>,
     #[cfg(feature = "webgpu")] wgpu_image_map: WGPUImageMap,
     protocols: ProtocolRegistry,
@@ -1105,7 +1106,6 @@ fn create_constellation(
     Constellation::<script::ScriptThread, script::ServiceWorkerManager>::start(
         initial_state,
         layout_factory,
-        initial_window_size,
         opts.random_pipeline_closure_probability,
         opts.random_pipeline_closure_seed,
         opts.hard_fail,
diff --git a/components/shared/constellation/lib.rs b/components/shared/constellation/lib.rs
index 38013398dd6..1861de20e52 100644
--- a/components/shared/constellation/lib.rs
+++ b/components/shared/constellation/lib.rs
@@ -17,17 +17,18 @@ use base::Epoch;
 use base::cross_process_instant::CrossProcessInstant;
 use base::id::{PipelineId, ScrollTreeNodeId, WebViewId};
 use bitflags::bitflags;
-use embedder_traits::{Cursor, InputEvent, MediaSessionActionType, Theme, WebDriverCommandMsg};
-use euclid::{Scale, Size2D, Vector2D};
+use embedder_traits::{
+    Cursor, InputEvent, MediaSessionActionType, Theme, ViewportDetails, WebDriverCommandMsg,
+};
+use euclid::Vector2D;
 use ipc_channel::ipc::IpcSender;
 use malloc_size_of::malloc_size_of_is_0;
 use malloc_size_of_derive::MallocSizeOf;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use servo_url::ServoUrl;
 use strum_macros::IntoStaticStr;
-use style_traits::CSSPixel;
 use webrender_api::ExternalScrollId;
-use webrender_api::units::{DevicePixel, LayoutPixel};
+use webrender_api::units::LayoutPixel;
 
 /// Messages to the constellation.
 #[derive(IntoStaticStr)]
@@ -47,8 +48,8 @@ pub enum ConstellationMsg {
     ClearCache,
     /// Request to traverse the joint session history of the provided browsing context.
     TraverseHistory(WebViewId, TraversalDirection),
-    /// Inform the constellation of a window being resized.
-    WindowSize(WebViewId, WindowSizeData, WindowSizeType),
+    /// Inform the Constellation that a `WebView`'s [`ViewportDetails`] have changed.
+    ChangeViewportDetails(WebViewId, ViewportDetails, WindowSizeType),
     /// Inform the constellation of a theme change.
     ThemeChange(Theme),
     /// Requests that the constellation instruct layout to begin a new tick of the animation.
@@ -60,7 +61,7 @@ pub enum ConstellationMsg {
     /// A log entry, with the top-level browsing context id and thread name
     LogEntry(Option<WebViewId>, Option<String>, LogEntry),
     /// Create a new top level browsing context.
-    NewWebView(ServoUrl, WebViewId),
+    NewWebView(ServoUrl, WebViewId, ViewportDetails),
     /// Close a top level browsing context.
     CloseWebView(WebViewId),
     /// Panic a top level browsing context.
@@ -115,17 +116,6 @@ pub enum LogEntry {
     Warn(String),
 }
 
-/// Data about the window size.
-#[derive(Clone, Copy, Debug, Deserialize, MallocSizeOf, PartialEq, Serialize)]
-pub struct WindowSizeData {
-    /// The size of the initial layout viewport, before parsing an
-    /// <http://www.w3.org/TR/css-device-adapt/#initial-viewport>
-    pub initial_viewport: Size2D<f32, CSSPixel>,
-
-    /// The resolution of the window in dppx, not including any "pinch zoom" factor.
-    pub device_pixel_ratio: Scale<f32, CSSPixel, DevicePixel>,
-}
-
 /// The type of window size change.
 #[derive(Clone, Copy, Debug, Deserialize, Eq, MallocSizeOf, PartialEq, Serialize)]
 pub enum WindowSizeType {
diff --git a/components/shared/embedder/lib.rs b/components/shared/embedder/lib.rs
index f617b37d705..67ccd9887ae 100644
--- a/components/shared/embedder/lib.rs
+++ b/components/shared/embedder/lib.rs
@@ -19,6 +19,7 @@ use std::sync::Arc;
 
 use base::id::{PipelineId, WebViewId};
 use crossbeam_channel::Sender;
+use euclid::{Scale, Size2D};
 use http::{HeaderMap, Method, StatusCode};
 use ipc_channel::ipc::IpcSender;
 pub use keyboard_types::{KeyboardEvent, Modifiers};
@@ -29,8 +30,9 @@ use pixels::Image;
 use serde::{Deserialize, Serialize};
 use servo_url::ServoUrl;
 use strum_macros::IntoStaticStr;
+use style_traits::CSSPixel;
 use url::Url;
-use webrender_api::units::{DeviceIntPoint, DeviceIntRect, DeviceIntSize};
+use webrender_api::units::{DeviceIntPoint, DeviceIntRect, DeviceIntSize, DevicePixel};
 
 pub use crate::input_events::*;
 pub use crate::webdriver::*;
@@ -224,7 +226,6 @@ pub enum AllowOrDeny {
 }
 
 #[derive(Clone, Debug, Deserialize, Serialize)]
-
 pub struct SelectElementOption {
     /// A unique identifier for the option that can be used to select it.
     pub id: usize,
@@ -244,6 +245,18 @@ pub enum SelectElementOptionOrOptgroup {
     },
 }
 
+/// Data about a `WebView` or `<iframe>` viewport: its size and also the
+/// HiDPI scale factor to use when rendering the contents.
+#[derive(Clone, Copy, Debug, Default, Deserialize, MallocSizeOf, PartialEq, Serialize)]
+pub struct ViewportDetails {
+    /// The size of the layout viewport.
+    pub size: Size2D<f32, CSSPixel>,
+
+    /// The scale factor to use to account for HiDPI scaling. This does not take into account
+    /// any page or pinch zoom applied by the compositor to the contents.
+    pub hidpi_scale_factor: Scale<f32, CSSPixel, DevicePixel>,
+}
+
 #[derive(Deserialize, IntoStaticStr, Serialize)]
 pub enum EmbedderMsg {
     /// A status message to be displayed by the browser chrome.
@@ -275,7 +288,7 @@ pub enum EmbedderMsg {
     /// Whether or not to allow a pipeline to load a url.
     AllowNavigationRequest(WebViewId, PipelineId, ServoUrl),
     /// Whether or not to allow script to open a new tab/browser
-    AllowOpeningWebView(WebViewId, IpcSender<Option<WebViewId>>),
+    AllowOpeningWebView(WebViewId, IpcSender<Option<(WebViewId, ViewportDetails)>>),
     /// A webview was destroyed.
     WebViewClosed(WebViewId),
     /// A webview gained focus for keyboard events.
diff --git a/components/shared/script/lib.rs b/components/shared/script/lib.rs
index 5f9c0eec777..8b4f2ca9966 100644
--- a/components/shared/script/lib.rs
+++ b/components/shared/script/lib.rs
@@ -27,13 +27,13 @@ use base::id::{
 use bluetooth_traits::BluetoothRequest;
 use canvas_traits::webgl::WebGLPipeline;
 use constellation_traits::{
-    AnimationTickType, CompositorHitTestResult, ScrollState, WindowSizeData, WindowSizeType,
+    AnimationTickType, CompositorHitTestResult, ScrollState, WindowSizeType,
 };
 use crossbeam_channel::{RecvTimeoutError, Sender};
 use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg, WorkerId};
 use embedder_traits::input_events::InputEvent;
 use embedder_traits::user_content_manager::UserContentManager;
-use embedder_traits::{MediaSessionActionType, Theme, WebDriverScriptCommand};
+use embedder_traits::{MediaSessionActionType, Theme, ViewportDetails, WebDriverScriptCommand};
 use euclid::{Rect, Scale, Size2D, UnknownUnit};
 use http::{HeaderMap, Method};
 use ipc_channel::Error as IpcError;
@@ -176,8 +176,8 @@ pub struct NewLayoutInfo {
     pub opener: Option<BrowsingContextId>,
     /// Network request data which will be initiated by the script thread.
     pub load_data: LoadData,
-    /// Information about the initial window size.
-    pub window_size: WindowSizeData,
+    /// Initial [`ViewportDetails`] for this layout.
+    pub viewport_details: ViewportDetails,
 }
 
 /// When a pipeline is closed, should its browsing context be discarded too?
@@ -252,11 +252,11 @@ pub enum ScriptThreadMessage {
     /// Gives a channel and ID to a layout, as well as the ID of that layout's parent
     AttachLayout(NewLayoutInfo),
     /// Window resized.  Sends a DOM event eventually, but first we combine events.
-    Resize(PipelineId, WindowSizeData, WindowSizeType),
+    Resize(PipelineId, ViewportDetails, WindowSizeType),
     /// Theme changed.
     ThemeChange(PipelineId, Theme),
     /// Notifies script that window has been resized but to not take immediate action.
-    ResizeInactive(PipelineId, WindowSizeData),
+    ResizeInactive(PipelineId, ViewportDetails),
     /// Window switched from fullscreen mode.
     ExitFullScreen(PipelineId),
     /// Notifies the script that the document associated with this pipeline should 'unload'.
@@ -447,8 +447,8 @@ pub struct InitialScriptState {
     pub memory_profiler_sender: mem::ProfilerChan,
     /// A channel to the developer tools, if applicable.
     pub devtools_server_sender: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
-    /// Information about the initial window size.
-    pub window_size: WindowSizeData,
+    /// Initial [`ViewportDetails`] for the frame that is initiating this `ScriptThread`.
+    pub viewport_details: ViewportDetails,
     /// The ID of the pipeline namespace for this script thread.
     pub pipeline_namespace_id: PipelineNamespaceId,
     /// A ping will be sent on this channel once the script thread shuts down.
@@ -537,7 +537,7 @@ pub struct IFrameLoadInfoWithData {
     /// Sandbox type of this iframe
     pub sandbox: IFrameSandboxState,
     /// The initial viewport size for this iframe.
-    pub window_size: WindowSizeData,
+    pub viewport_details: ViewportDetails,
 }
 
 /// Resources required by workerglobalscopes
diff --git a/components/shared/script/script_msg.rs b/components/shared/script/script_msg.rs
index 8baf158312c..9d03ce2509c 100644
--- a/components/shared/script/script_msg.rs
+++ b/components/shared/script/script_msg.rs
@@ -13,8 +13,9 @@ use base::id::{
 use canvas_traits::canvas::{CanvasId, CanvasMsg};
 use constellation_traits::{LogEntry, TraversalDirection};
 use devtools_traits::{ScriptToDevtoolsControlMsg, WorkerId};
-use embedder_traits::{EmbedderMsg, MediaSessionEvent, TouchEventType, TouchSequenceId};
-use euclid::Size2D;
+use embedder_traits::{
+    EmbedderMsg, MediaSessionEvent, TouchEventType, TouchSequenceId, ViewportDetails,
+};
 use euclid::default::Size2D as UntypedSize2D;
 use ipc_channel::ipc::{IpcReceiver, IpcSender};
 use net_traits::CoreResourceMsg;
@@ -22,7 +23,6 @@ use net_traits::storage_thread::StorageType;
 use serde::{Deserialize, Serialize};
 use servo_url::{ImmutableOrigin, ServoUrl};
 use strum_macros::IntoStaticStr;
-use style_traits::CSSPixel;
 #[cfg(feature = "webgpu")]
 use webgpu_traits::{WebGPU, WebGPUAdapterResponse};
 use webrender_api::ImageKey;
@@ -39,8 +39,8 @@ use crate::{
 pub struct IFrameSizeMsg {
     /// The child browsing context for this iframe.
     pub browsing_context_id: BrowsingContextId,
-    /// The size of the iframe.
-    pub size: Size2D<f32, CSSPixel>,
+    /// The size and scale factor of the iframe.
+    pub size: ViewportDetails,
     /// The kind of sizing operation.
     pub type_: WindowSizeType,
 }
diff --git a/components/shared/script_layout/Cargo.toml b/components/shared/script_layout/Cargo.toml
index 13d50e4fafe..5c0b6e01d36 100644
--- a/components/shared/script_layout/Cargo.toml
+++ b/components/shared/script_layout/Cargo.toml
@@ -17,6 +17,7 @@ app_units = { workspace = true }
 atomic_refcell = { workspace = true }
 canvas_traits = { workspace = true }
 constellation_traits = { workspace = true }
+embedder_traits = { workspace = true }
 euclid = { workspace = true }
 fnv = { workspace = true }
 fonts = { path = "../../fonts" }
diff --git a/components/shared/script_layout/lib.rs b/components/shared/script_layout/lib.rs
index 817ddd820bd..66ddb3a55fa 100644
--- a/components/shared/script_layout/lib.rs
+++ b/components/shared/script_layout/lib.rs
@@ -18,8 +18,8 @@ use app_units::Au;
 use atomic_refcell::AtomicRefCell;
 use base::Epoch;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
-use constellation_traits::{ScrollState, UntrustedNodeAddress, WindowSizeData};
-use euclid::Size2D;
+use constellation_traits::{ScrollState, UntrustedNodeAddress};
+use embedder_traits::ViewportDetails;
 use euclid::default::{Point2D, Rect};
 use fnv::FnvHashMap;
 use fonts::{FontContext, SystemFontServiceProxy};
@@ -47,7 +47,6 @@ use style::properties::style_structs::Font;
 use style::queries::values::PrefersColorScheme;
 use style::selector_parser::{PseudoElement, RestyleDamage, Snapshot};
 use style::stylesheets::Stylesheet;
-use style_traits::CSSPixel;
 use webrender_api::ImageKey;
 use webrender_traits::CrossProcessCompositorApi;
 
@@ -185,7 +184,7 @@ pub struct LayoutConfig {
     pub font_context: Arc<FontContext>,
     pub time_profiler_chan: time::ProfilerChan,
     pub compositor_api: CrossProcessCompositorApi,
-    pub window_size: WindowSizeData,
+    pub viewport_details: ViewportDetails,
 }
 
 pub trait LayoutFactory: Send + Sync {
@@ -386,7 +385,7 @@ pub struct Reflow {
 pub struct IFrameSize {
     pub browsing_context_id: BrowsingContextId,
     pub pipeline_id: PipelineId,
-    pub size: Size2D<f32, CSSPixel>,
+    pub viewport_details: ViewportDetails,
 }
 
 pub type IFrameSizes = FnvHashMap<BrowsingContextId, IFrameSize>;
@@ -415,8 +414,8 @@ pub struct ReflowRequest {
     pub dirty_root: Option<TrustedNodeAddress>,
     /// Whether the document's stylesheets have changed since the last script reflow.
     pub stylesheets_changed: bool,
-    /// The current window size.
-    pub window_size: WindowSizeData,
+    /// The current [`ViewportDetails`] to use for this reflow.
+    pub viewport_details: ViewportDetails,
     /// The goal of this reflow.
     pub reflow_goal: ReflowGoal,
     /// The number of objects in the dom #10110