Auto merge of #17982 - heycam:parent-fixup, r=emilio

style: Don't perform parent display-based style fixups on most pseudos.

Reviewed by Emilio in https://bugzilla.mozilla.org/show_bug.cgi?id=1379865.

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components/style/gecko/pseudo_element.rs

@@ -115,6 +115,13 @@ impl PseudoElement {
         (self.flags() & structs::CSS_PSEUDO_ELEMENT_SUPPORTS_USER_ACTION_STATE) != 0
     }
 
+    /// Whether this pseudo-element skips flex/grid container display-based
+    /// fixup.
+    #[inline]
+    pub fn skip_item_based_display_fixup(&self) -> bool {
+        (self.flags() & structs::CSS_PSEUDO_ELEMENT_IS_FLEX_OR_GRID_ITEM) == 0
+    }
+
     /// Whether this pseudo-element is precomputed.
     #[inline]
     pub fn is_precomputed(&self) -> bool {

components/style/servo/selector_parser.rs

@@ -150,6 +150,13 @@ impl PseudoElement {
         self.is_precomputed()
     }
 
+    /// Whether this pseudo-element skips flex/grid container
+    /// display-based fixup.
+    #[inline]
+    pub fn skip_item_based_display_fixup(&self) -> bool {
+        !self.is_before_or_after()
+    }
+
     /// Whether this pseudo-element is precomputed.
     #[inline]
     pub fn is_precomputed(&self) -> bool {

components/style/style_resolver.rs

@@ -442,7 +442,8 @@ where
         let mut cascade_info = CascadeInfo::new();
         let mut cascade_flags = CascadeFlags::empty();
 
-        if self.element.skip_root_and_item_based_display_fixup() {
+        if self.element.skip_root_and_item_based_display_fixup() ||
+           pseudo.map_or(false, |p| p.skip_item_based_display_fixup()) {
             cascade_flags.insert(SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP);
         }