Bug 1184842. Restyling should consider only the classes that have changed. r=heycam

--HG--
extra : commitid : 4MDNftjpZQx
extra : rebase_source : e2951539cbd0b6a8e0a9cbced287bf3820572728

layout/style/nsCSSRuleProcessor.cpp

@@ -55,6 +55,7 @@
 #include "mozilla/Likely.h"
 #include "mozilla/TypedEnumBits.h"
 #include "RuleProcessorCache.h"
+#include "nsIDOMMutationEvent.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -2802,17 +2803,39 @@ nsCSSRuleProcessor::HasAttributeDependentStyle(AttributeRuleProcessorData* aData
     }
 
     if (aData->mAttribute == nsGkAtoms::_class) {
+      const nsAttrValue* otherClasses = aData->mOtherValue;
+      NS_ASSERTION(otherClasses ||
+                   aData->mModType == nsIDOMMutationEvent::REMOVAL,
+                   "All class values should be StoresOwnData and parsed"
+                   "via Element::BeforeSetAttr, so available here");
+      // For WillChange, enumerate classes that will be removed to see which
+      // rules apply before the change.
+      // For Changed, enumerate classes that have been added to see which rules
+      // apply after the change.
+      // In both cases we're interested in the classes that are currently on
+      // the element but not in mOtherValue.
       const nsAttrValue* elementClasses = aData->mElement->GetClasses();
       if (elementClasses) {
         int32_t atomCount = elementClasses->GetAtomCount();
-        for (int32_t i = 0; i < atomCount; ++i) {
-          nsIAtom* curClass = elementClasses->AtomAt(i);
-          AtomSelectorEntry *entry =
-            static_cast<AtomSelectorEntry*>
-                       (PL_DHashTableSearch(&cascade->mClassSelectors,
-                                            curClass));
-          if (entry) {
-            EnumerateSelectors(entry->mSelectors, &data);
+        if (atomCount > 0) {
+          nsTHashtable<nsPtrHashKey<nsIAtom>> otherClassesTable;
+          if (otherClasses) {
+            int32_t otherClassesCount = otherClasses->GetAtomCount();
+            for (int32_t i = 0; i < otherClassesCount; ++i) {
+              otherClassesTable.PutEntry(otherClasses->AtomAt(i));
+            }
+          }
+          for (int32_t i = 0; i < atomCount; ++i) {
+            nsIAtom* curClass = elementClasses->AtomAt(i);
+            if (!otherClassesTable.Contains(curClass)) {
+              AtomSelectorEntry *entry =
+                static_cast<AtomSelectorEntry*>
+                           (PL_DHashTableSearch(&cascade->mClassSelectors,
+                                                curClass));
+              if (entry) {
+                EnumerateSelectors(entry->mSelectors, &data);
+              }
+            }
           }
         }
       }