Bug 1460295 - Don't dispatch mutation event for style attribute change from CSSOM. r=bz

MozReview-Commit-ID: JWDpyg7czXI

--HG--
extra : rebase_source : 1bf116ff3c016da126dae7ba89282cf31f16660a

dom/base/nsStyledElement.cpp

@@ -12,6 +12,7 @@
 #include "mozilla/dom/ElementInlines.h"
 #include "mozilla/dom/MutationEventBinding.h"
 #include "mozilla/InternalMutationEvent.h"
+#include "mozilla/StaticPrefs.h"
 #include "nsDOMCSSDeclaration.h"
 #include "nsDOMCSSAttrDeclaration.h"
 #include "nsServiceManagerUtils.h"
@@ -77,6 +78,7 @@ nsStyledElement::SetInlineStyleDeclaration(DeclarationBlock* aDeclaration,
   bool oldValueSet = false;
 
   bool hasListeners = aNotify &&
+    !StaticPrefs::dom_mutation_events_cssom_disabled() &&
     nsContentUtils::HasMutationListeners(this,
                                          NS_EVENT_BITS_MUTATION_ATTRMODIFIED,
                                          this);

dom/base/test/test_bug338679.html

@@ -35,9 +35,13 @@ function nextTest() {
   }
 }
 
-SimpleTest.waitForExplicitFinish();
-testDiv.addEventListener("DOMAttrModified", attr_modified);
-nextTest();
+if (SpecialPowers.getBoolPref("dom.mutation-events.cssom.disabled")) {
+  ok(true, "DOMAttrModified event from CSSOM change is disabled");
+} else {
+  SimpleTest.waitForExplicitFinish();
+  testDiv.addEventListener("DOMAttrModified", attr_modified);
+  nextTest();
+}
 
 /* event handler */
 function attr_modified(ev) {

layout/style/test/test_style_attr_listener.html

@@ -21,6 +21,11 @@ expectation = { prevValue: 'color: red;', newValue: 'color: green;' };
 div.style.color = "green";
 expectation = { prevValue: 'color: green;', newValue: '' };
 div.style.color = '';
+if (SpecialPowers.getBoolPref("dom.mutation-events.cssom.disabled")) {
+  is(count, 0, "No DOMAttrModified event should be triggered");
+} else {
+  is(count, 2, "DOMAttrModified events should have been triggered");
+}
 </script>
 </body>
 </html>

modules/libpref/init/StaticPrefList.h

@@ -97,6 +97,14 @@ VARCACHE_PREF(
   bool, false
 )
 
+// Whether we disable triggering mutation events for changes to style
+// attribute via CSSOM.
+VARCACHE_PREF(
+  "dom.mutation-events.cssom.disabled",
+   dom_mutation_events_cssom_disabled,
+  bool, true
+)
+
 //---------------------------------------------------------------------------
 // Full-screen prefs
 //---------------------------------------------------------------------------