Use the size of the containing block, not the size of the block formatting

context, to place floats in layout 2020. The containing block for a float is not necessarily the same as the block formatting context the float is in per CSS 2.1 [1]: "For other elements, if the element’s position is relative or static, the containing block is formed by the content edge of the nearest block container ancestor box." This shows up in the simplest case: <html> <body> <div style="float: left">Hello</div> </body> </html> In this case, the `<html>` element is the block formatting context with inline size equal to the width of the window, but the `<body>` element with nonzero inline margins is the containing block for the float. The float placement must respect the content box of the `<body>` element (i.e. floats must not overlap the `<body>` element's margins), not that of the `<html>` element. Because a single block formatting context may contain floats with different containing blocks, the left and right "walls" of that containing block become properties of individual floats at the time of placement, not properties of the float context itself. Additionally, this commit generalizes the float placement logic a bit to allow the placement of arbitrary objects, not just floats. This is intended to support inline layout and block formatting context placement. This commit updates the `FloatContext` and associated tests only and doesn't actually wire the context up to the rest of layout, so floats in pages still aren't actually laid out. [1]: https://drafts.csswg.org/css2/#containing-block-details
author: Patrick Walton <pcwalton@mimiga.net> 2020-07-22 19:07:36 -0700
committer: Patrick Walton <pcwalton@mimiga.net> 2020-07-22 19:58:28 -0700
commit: 362b64aa68f256016506e82747edb846f93d371a (patch)
tree: 994e0859242f91014bc2af1d3cf4b9c2f3d19a04 /components/layout_2020/flow
parent: 6a9aac3e654d4498c8c2605ebe8a24b609770e21 (diff)
download: servo-362b64aa68f256016506e82747edb846f93d371a.tar.gz
servo-362b64aa68f256016506e82747edb846f93d371a.zip
2 files changed, 106 insertions, 53 deletions
diff --git a/components/layout_2020/flow/float.rs b/components/layout_2020/flow/float.rs
index 5cb349310e3..d7269cbfc4c 100644
--- a/components/layout_2020/flow/float.rs
+++ b/components/layout_2020/flow/float.rs
@@ -36,9 +36,6 @@ pub struct FloatContext {
     /// This tree is immutable; modification operations return the new tree, which may share nodes
     /// with previous versions of the tree.
     pub bands: FloatBandTree,
-    /// The logical width of this context. No floats may extend outside the width of this context
-    /// unless they are as far (logically) left or right as possible.
-    pub inline_size: Length,
     /// The current (logically) vertical position. No new floats may be placed (logically) above
     /// this line.
     pub ceiling: Length,
@@ -47,7 +44,7 @@ pub struct FloatContext {
 impl FloatContext {
     /// Returns a new float context representing a containing block with the given content
     /// inline-size.
-    pub fn new(inline_size: Length) -> Self {
+    pub fn new() -> Self {
         let mut bands = FloatBandTree::new();
         bands = bands.insert(FloatBand {
             top: Length::zero(),
@@ -61,7 +58,6 @@ impl FloatContext {
         });
         FloatContext {
             bands,
-            inline_size,
             ceiling: Length::zero(),
         }
     }
@@ -78,25 +74,15 @@ impl FloatContext {
         self.ceiling = self.ceiling.max(new_ceiling);
     }
 
-    /// Returns the highest block position that is both logically below the current ceiling and
-    /// clear of floats on the given side or sides.
-    pub fn clearance(&self, side: ClearSide) -> Length {
-        let mut band = self.bands.find(self.ceiling).unwrap();
-        while !band.is_clear(side) {
-            let next_band = self.bands.find_next(band.top).unwrap();
-            if next_band.top.px().is_infinite() {
-                break;
-            }
-            band = next_band;
-        }
-        band.top.max(self.ceiling)
-    }
-
-    /// Places a new float and adds it to the list. Returns the start corner of its margin box.
-    pub fn add_float(&mut self, new_float: FloatInfo) -> Vec2<Length> {
+    /// Determines where a float with the given placement would go, but leaves the float context
+    /// unmodified. Returns the start corner of its margin box.
+    ///
+    /// This should be used for placing inline elements and block formatting contexts so that they
+    /// don't collide with floats.
+    pub fn place_object(&self, object: &PlacementInfo) -> Vec2<Length> {
         // Find the first band this float fits in.
         let mut first_band = self.bands.find(self.ceiling).unwrap();
-        while !first_band.float_fits(&new_float, self.inline_size) {
+        while !first_band.object_fits(&object) {
             let next_band = self.bands.find_next(first_band.top).unwrap();
             if next_band.top.px().is_infinite() {
                 break;
@@ -104,30 +90,46 @@ impl FloatContext {
             first_band = next_band;
         }
 
-        // The float fits perfectly here. Place it.
-        let (new_float_origin, new_float_extent);
-        match new_float.side {
+        // The object fits perfectly here. Place it.
+        match object.side {
             FloatSide::Left => {
-                new_float_origin = Vec2 {
-                    inline: first_band.left.unwrap_or(Length::zero()),
-                    block: first_band.top.max(self.ceiling),
+                let left_object_edge = match first_band.left {
+                    Some(band_left) => band_left.max(object.left_wall),
+                    None => object.left_wall,
                 };
-                new_float_extent = new_float_origin.inline + new_float.size.inline;
+                Vec2 {
+                    inline: left_object_edge,
+                    block: first_band.top.max(self.ceiling),
+                }
             },
             FloatSide::Right => {
-                new_float_origin = Vec2 {
-                    inline: first_band.right.unwrap_or(self.inline_size) - new_float.size.inline,
-                    block: first_band.top.max(self.ceiling),
+                let right_object_edge = match first_band.right {
+                    Some(band_right) => band_right.min(object.right_wall),
+                    None => object.right_wall,
                 };
-                new_float_extent = new_float_origin.inline;
+                Vec2 {
+                    inline: right_object_edge - object.size.inline,
+                    block: first_band.top.max(self.ceiling),
+                }
             },
+        }
+    }
+
+    /// Places a new float and adds it to the list. Returns the start corner of its margin box.
+    pub fn add_float(&mut self, new_float: &PlacementInfo) -> Vec2<Length> {
+        // Place the float.
+        let new_float_origin = self.place_object(new_float);
+        let new_float_extent = match new_float.side {
+            FloatSide::Left => new_float_origin.inline + new_float.size.inline,
+            FloatSide::Right => new_float_origin.inline,
         };
         let new_float_rect = Rect {
             start_corner: new_float_origin,
-            size: new_float.size,
+            size: new_float.size.clone(),
         };
 
         // Split the first band if necessary.
+        let mut first_band = self.bands.find(new_float_rect.start_corner.block).unwrap();
         first_band.top = new_float_rect.start_corner.block;
         self.bands = self.bands.insert(first_band);
 
@@ -152,15 +154,21 @@ impl FloatContext {
     }
 }
 
-/// Information needed to place a float.
+/// Information needed to place an object so that it doesn't collide with existing floats.
 #[derive(Clone, Debug)]
-pub struct FloatInfo {
-    /// The *margin* box size of the float.
+pub struct PlacementInfo {
+    /// The *margin* box size of the object.
     pub size: Vec2<Length>,
-    /// Whether the float is left or right.
+    /// Whether the object is (logically) aligned to the left or right.
     pub side: FloatSide,
     /// Which side or sides to clear floats on.
     pub clear: ClearSide,
+    /// The distance from the logical left side of the block formatting context to the logical
+    /// left side of this object's containing block.
+    pub left_wall: Length,
+    /// The distance from the logical *left* side of the block formatting context to the logical
+    /// right side of this object's containing block.
+    pub right_wall: Length,
 }
 
 /// Whether the float is left or right.
@@ -189,19 +197,20 @@ pub enum ClearSide {
 pub struct FloatBand {
     /// The logical vertical position of the top of this band.
     pub top: Length,
-    /// The distance from the left edge to the first legal (logically) horizontal position where
-    /// floats may be placed. If `None`, there are no floats to the left; distinguishing between
-    /// the cases of "a zero-width float is present" and "no floats at all are present" is
-    /// necessary to, for example, clear past zero-width floats.
+    /// The distance from the left edge of the block formatting context to the first legal
+    /// (logically) horizontal position where floats may be placed. If `None`, there are no floats
+    /// to the left; distinguishing between the cases of "a zero-width float is present" and "no
+    /// floats at all are present" is necessary to, for example, clear past zero-width floats.
     pub left: Option<Length>,
-    /// The distance from the right edge to the first legal (logically) horizontal position where
-    /// floats may be placed. If `None`, there are no floats to the right; distinguishing between
-    /// the cases of "a zero-width float is present" and "no floats at all are present" is
-    /// necessary to, for example, clear past zero-width floats.
+    /// The distance from the *left* edge of the block formatting context to the first legal
+    /// (logically) horizontal position where floats may be placed. If `None`, there are no floats
+    /// to the right; distinguishing between the cases of "a zero-width float is present" and "no
+    /// floats at all are present" is necessary to, for example, clear past zero-width floats.
     pub right: Option<Length>,
 }
 
 impl FloatBand {
+    // Returns true if this band is clear of floats on the given side or sides.
     fn is_clear(&self, side: ClearSide) -> bool {
         match (side, self.left, self.right) {
             (ClearSide::Left, Some(_), _) |
@@ -215,12 +224,56 @@ impl FloatBand {
         }
     }
 
-    fn float_fits(&self, new_float: &FloatInfo, container_inline_size: Length) -> bool {
-        let available_space =
-            self.right.unwrap_or(container_inline_size) - self.left.unwrap_or(Length::zero());
-        self.is_clear(new_float.clear) &&
-            (new_float.size.inline <= available_space ||
-                (self.left.is_none() && self.right.is_none()))
+    // Determines whether an object fits in a band.
+    fn object_fits(&self, object: &PlacementInfo) -> bool {
+        // If we must be clear on the given side and we aren't, this object doesn't fit.
+        if !self.is_clear(object.clear) {
+            return false;
+        }
+
+        match object.side {
+            FloatSide::Left => {
+                // Compute a candidate left position for the object.
+                let candidate_left = match self.left {
+                    None => object.left_wall,
+                    Some(left) => left.max(object.left_wall),
+                };
+
+                // If this band has an existing left float in it, then make sure that the object
+                // doesn't stick out past the right edge (rule 7).
+                if self.left.is_some() && candidate_left + object.size.inline > object.right_wall {
+                    return false;
+                }
+
+                // If this band has an existing right float in it, make sure we don't collide with
+                // it (rule 3).
+                match self.right {
+                    None => true,
+                    Some(right) => object.size.inline <= right - candidate_left,
+                }
+            },
+
+            FloatSide::Right => {
+                // Compute a candidate right position for the object.
+                let candidate_right = match self.right {
+                    None => object.right_wall,
+                    Some(right) => right.min(object.right_wall),
+                };
+
+                // If this band has an existing right float in it, then make sure that the new
+                // object doesn't stick out past the left edge (rule 7).
+                if self.right.is_some() && candidate_right - object.size.inline < object.left_wall {
+                    return false;
+                }
+
+                // If this band has an existing left float in it, make sure we don't collide with
+                // it (rule 3).
+                match self.left {
+                    None => true,
+                    Some(left) => object.size.inline <= candidate_right - left,
+                }
+            },
+        }
     }
 }
 
diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index f042f814dd3..b5898fc6146 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -80,7 +80,7 @@ impl BlockFormattingContext {
     ) -> IndependentLayout {
         let mut float_context;
         let float_context = if self.contains_floats {
-            float_context = FloatContext::new(containing_block.inline_size);
+            float_context = FloatContext::new();
             Some(&mut float_context)
         } else {
             None
author	Patrick Walton <pcwalton@mimiga.net>	2020-07-22 19:07:36 -0700
committer	Patrick Walton <pcwalton@mimiga.net>	2020-07-22 19:58:28 -0700
commit	362b64aa68f256016506e82747edb846f93d371a (patch)
tree	994e0859242f91014bc2af1d3cf4b9c2f3d19a04 /components/layout_2020/flow
parent	6a9aac3e654d4498c8c2605ebe8a24b609770e21 (diff)
download	servo-362b64aa68f256016506e82747edb846f93d371a.tar.gz servo-362b64aa68f256016506e82747edb846f93d371a.zip