Don't treat event regions as opaque for plugin geometry. (bug 1109424, r=roc)

layout/base/nsDisplayList.cpp

@@ -1338,7 +1338,9 @@ TreatAsOpaque(nsDisplayItem* aItem, nsDisplayListBuilder* aBuilder)
 {
   bool snap;
   nsRegion opaque = aItem->GetOpaqueRegion(aBuilder, &snap);
-  if (aBuilder->IsForPluginGeometry()) {
+  if (aBuilder->IsForPluginGeometry() &&
+      aItem->GetType() != nsDisplayItem::TYPE_LAYER_EVENT_REGIONS)
+  {
     // Treat all leaf chrome items as opaque, unless their frames are opacity:0.
     // Since opacity:0 frames generate an nsDisplayOpacity, that item will
     // not be treated as opaque here, so opacity:0 chrome content will be

layout/base/tests/chrome/test_chrome_over_plugin.xul

@@ -16,6 +16,7 @@
     SimpleTest.waitForExplicitFinish();
     // Run the test in a separate window so that the test runs as a chrome
     // window
+    SpecialPowers.setTestPluginEnabledState(SpecialPowers.Ci.nsIPluginTag.STATE_ENABLED);
     var w = window.open("chrome_over_plugin_window.xul", "chrome_over_plugin",
                          "chrome,width=200,height=300");
   ]]>