Auto merge of #11035 - mrobinson:no-stacking-contexts-for-text-fragments, r=pcwalton

Do not create stacking contexts for text fragments

Without this change, each text fragment in a block that establishes a
stacking context will establish its own stacking context. This is
unnecessary and increases the amount of work done during display list
construction. This change should not change output, but should improve
performance.

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components/layout/fragment.rs

@@ -2207,6 +2207,13 @@ impl Fragment {
 
     /// Returns true if this fragment establishes a new stacking context and false otherwise.
     pub fn establishes_stacking_context(&self) -> bool {
+        // Text fragments shouldn't create stacking contexts.
+        match self.specific {
+            SpecificFragmentInfo::ScannedText(_) |
+            SpecificFragmentInfo::UnscannedText(_) => return false,
+            _ => {}
+        }
+
         if self.flags.contains(HAS_LAYER) {
             return true
         }