Auto merge of #29644 - mrobinson:rename-establishes-containing-block, r=delan

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.

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---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they do not change behavior.

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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()