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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- [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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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;
         }
 

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(),

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()