Improve logic for establishing a stacking context (#35947)

In particular:
 - `z-index` will now work on unpositioned grid items.
 - `will-change: z-index` will only establish a stacking context if
   `z-index` applies, i.e. if the box is positioned or a flex/grid item.
 - The conditions in `establishes_stacking_context()` are reordered,
   so that the most likely ones are checked first.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/components/layout_2020/fragment_tree/base_fragment.rs b/components/layout_2020/fragment_tree/base_fragment.rs
index 39d06ca121a..38f4f6ba815 100644
--- a/components/layout_2020/fragment_tree/base_fragment.rs
+++ b/components/layout_2020/fragment_tree/base_fragment.rs
@@ -80,8 +80,8 @@ bitflags! {
         const IS_BODY_ELEMENT_OF_HTML_ELEMENT_ROOT = 1 << 0;
         /// Whether or not the node that created this Fragment is a `<br>` element.
         const IS_BR_ELEMENT = 1 << 1;
-        /// Whether or not this Fragment is a flex item.
-        const IS_FLEX_ITEM = 1 << 2;
+        /// Whether or not this Fragment is a flex item or a grid item.
+        const IS_FLEX_OR_GRID_ITEM = 1 << 2;
         /// Whether or not this Fragment was created to contain a replaced element or is
         /// a replaced element.
         const IS_REPLACED = 1 << 3;