Bug 1359834 Part 2: Define a new nsChangeHint_ReflowHintsForFloatAreaChange hint and use it for float changes. r=dholbert

MozReview-Commit-ID: 692GBiRUTab

--HG--
extra : rebase_source : 4b3413bb0468037ab5eb25101b7c2a54cdc63d9e

layout/base/nsChangeHint.h

@@ -427,6 +427,17 @@ static_assert(!(nsChangeHint_Hints_AlwaysHandledForDescendants &
                ~(nsChangeHint_ClearDescendantIntrinsics | \
                  nsChangeHint_NeedDirtyReflow))
 
+// * For changes to the float area of an already-floated element, we need all
+// reflow hints, but not the ones that apply to descendants.
+// Our descendants aren't impacted when our float area only changes
+// placement but not size/shape. (e.g. if we change which side we float to).
+// But our ancestors/siblings are potentially impacted, so we need to send
+// the non-descendant reflow hints.
+#define nsChangeHint_ReflowHintsForFloatAreaChange            \
+  nsChangeHint(nsChangeHint_AllReflowHints &              \
+               ~(nsChangeHint_ClearDescendantIntrinsics | \
+                 nsChangeHint_NeedDirtyReflow))
+
 #define NS_STYLE_HINT_REFLOW \
   nsChangeHint(NS_STYLE_HINT_VISUAL | nsChangeHint_AllReflowHints)
 

layout/style/nsStyleStruct.cpp

@@ -3527,10 +3527,8 @@ nsStyleDisplay::CalcDifference(const nsStyleDisplay& aNewData) const
    */
 
   if (mFloat != aNewData.mFloat) {
-    // Changing which side we float on doesn't affect descendants directly
-    hint |= nsChangeHint_AllReflowHints &
-            ~(nsChangeHint_ClearDescendantIntrinsics |
-              nsChangeHint_NeedDirtyReflow);
+    // Changing which side we're floating on (float:none was handled above).
+    hint |= nsChangeHint_ReflowHintsForFloatAreaChange;
   }
 
   if (mVerticalAlign != aNewData.mVerticalAlign) {