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.

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()