Bug 732667. Only do checks for :hover selectors when hover state changes on nodes which have relevant hover rules. r=dbaron

content/base/public/nsINode.h

@@ -175,11 +175,14 @@ enum {
   // Set if the node is handling a click.
   NODE_HANDLING_CLICK          = 0x00040000U,
 
+  // Set if the node has had :hover selectors matched against it
+  NODE_HAS_RELEVANT_HOVER_RULES = 0x00080000U,
+
   // Two bits for the script-type ID.  Not enough to represent all
   // nsIProgrammingLanguage values, but we don't care.  In practice,
   // we can represent the ones we want, and we can fail the others at
   // runtime.
-  NODE_SCRIPT_TYPE_OFFSET =               19,
+  NODE_SCRIPT_TYPE_OFFSET =               20,
 
   NODE_SCRIPT_TYPE_SIZE =                  2,
 

layout/base/nsCSSFrameConstructor.cpp

@@ -8085,8 +8085,14 @@ nsCSSFrameConstructor::ContentStateChanged(nsIContent* aContent,
     primaryFrame->ContentStatesChanged(aStateMask);
   }
 
-  nsRestyleHint rshint = 
-    styleSet->HasStateDependentStyle(presContext, aElement, aStateMask);
+  if (aStateMask.HasState(NS_EVENT_STATE_HOVER) &&
+      !aElement->HasFlag(NODE_HAS_RELEVANT_HOVER_RULES)) {
+    aStateMask &= ~NS_EVENT_STATE_HOVER;
+  }
+
+  nsRestyleHint rshint = aStateMask.IsEmpty() ?
+      nsRestyleHint(0) :
+      styleSet->HasStateDependentStyle(presContext, aElement, aStateMask);
       
   if (aStateMask.HasState(NS_EVENT_STATE_HOVER) && rshint != 0) {
     ++mHoverGeneration;

layout/style/nsCSSRuleProcessor.cpp

@@ -2114,6 +2114,11 @@ static bool SelectorMatches(Element* aElement,
         // selectors ":hover" and ":active".
         return false;
       } else {
+        if (aTreeMatchContext.mForStyling &&
+            statesToCheck.HasAtLeastOneOfStates(NS_EVENT_STATE_HOVER)) {
+          // Mark the element as having :hover-dependent style
+          aElement->SetFlags(NODE_HAS_RELEVANT_HOVER_RULES);
+        }
         if (aNodeMatchContext.mStateMask.HasAtLeastOneOfStates(statesToCheck)) {
           if (aDependence)
             *aDependence = true;