Rename `ComputedValuesExt::establishes_containing_block`

This renames the helper method to be a bit more accurate. For elements
with static, relative, and sticky positioning, their containing block is
always formed by their nearest block container ancestor. This method is
really dealing with style that means an element will establish a
containing block for absolutely positioned descendants.

3 files changed, 17 insertions, 6 deletions

diff --git a/components/layout_2020/display_list/stacking_context.rs b/components/layout_2020/display_list/stacking_context.rs
index 78992d9c5bc..8cc717569f9 100644
--- a/components/layout_2020/display_list/stacking_context.rs
+++ b/components/layout_2020/display_list/stacking_context.rs
@@ -540,7 +540,10 @@ impl BoxFragment {
         padding_rect: &PhysicalRect<Length>,
         containing_block_info: &mut ContainingBlockInfo,
     ) {
-        if !self.style.establishes_containing_block() {
+        if !self
+            .style
+            .establishes_containing_block_for_absolute_descendants()
+        {
             return;
         }
 
diff --git a/components/layout_2020/positioned.rs b/components/layout_2020/positioned.rs
index 2580d4af9bc..883f783dfd8 100644
--- a/components/layout_2020/positioned.rs
+++ b/components/layout_2020/positioned.rs
@@ -196,7 +196,7 @@ impl PositioningContext {
     pub(crate) fn new_for_style(style: &ComputedValues) -> Option<Self> {
         if style.establishes_containing_block_for_all_descendants() {
             Some(Self::new_for_containing_block_for_all_descendants())
-        } else if style.establishes_containing_block() {
+        } else if style.establishes_containing_block_for_absolute_descendants() {
             Some(Self {
                 for_nearest_positioned_ancestor: Some(Vec::new()),
                 for_nearest_containing_block_for_all_descendants: Vec::new(),
diff --git a/components/layout_2020/style_ext.rs b/components/layout_2020/style_ext.rs
index 58551aedbe6..c1d2f47e2fa 100644
--- a/components/layout_2020/style_ext.rs
+++ b/components/layout_2020/style_ext.rs
@@ -113,7 +113,7 @@ pub(crate) trait ComputedValuesExt {
     fn has_transform_or_perspective(&self) -> bool;
     fn effective_z_index(&self) -> i32;
     fn establishes_stacking_context(&self) -> bool;
-    fn establishes_containing_block(&self) -> bool;
+    fn establishes_containing_block_for_absolute_descendants(&self) -> bool;
     fn establishes_containing_block_for_all_descendants(&self) -> bool;
     fn background_is_transparent(&self) -> bool;
     fn get_webrender_primitive_flags(&self) -> wr::PrimitiveFlags;
@@ -395,7 +395,13 @@ impl ComputedValuesExt for ComputedValues {
         !self.get_position().z_index.is_auto()
     }
 
-    fn establishes_containing_block(&self) -> bool {
+    /// Returns true if this style establishes a containing block for absolute
+    /// descendants (`position: absolute`). If this style happens to establish a
+    /// containing block for “all descendants” (ie including `position: fixed`
+    /// descendants) this method will return true, but a true return value does
+    /// not imply that the style establishes a containing block for all descendants.
+    /// Use `establishes_containing_block_for_all_descendants()` instead.
+    fn establishes_containing_block_for_absolute_descendants(&self) -> bool {
         if self.establishes_containing_block_for_all_descendants() {
             return true;
         }
@@ -403,8 +409,10 @@ impl ComputedValuesExt for ComputedValues {
         self.clone_position() != ComputedPosition::Static
     }
 
-    /// Returns true if this style establishes a containing block for all descendants
-    /// including fixed and absolutely positioned ones.
+    /// Returns true if this style establishes a containing block for
+    /// all descendants, including fixed descendants (`position: fixed`).
+    /// Note that this also implies that it establishes a containing block
+    /// for absolute descendants (`position: absolute`).
     fn establishes_containing_block_for_all_descendants(&self) -> bool {
         if self.get_box().display.outside() != stylo::DisplayOutside::Inline &&
             self.has_transform_or_perspective()