Bug 656749. Only optimize away zero-opacity elements when we're painting, not for any other kind of display list construction such as plugin geometry. r=tnikkel

dom/plugins/test/mochitest/Makefile.in

@@ -99,6 +99,7 @@ _MOCHITEST_FILES = \
   307-xo-redirect.sjs \
   test_redirect_handling.html \
   test_clear_site_data.html \
+  test_zero_opacity.html \
   $(NULL)
 
 #  test_plugin_scroll_painting.html \ bug 596491

dom/plugins/test/mochitest/test_zero_opacity.html

@@ -17,6 +17,12 @@
   var p = document.getElementById('theplugin');
 
   function startTest() {
+    if (!p.hasWidget()) {
+      todo(false, "This test is only relevant for windowed plugins");
+      SimpleTest.finish();
+      return;
+    }
+
     // Wait for the plugin to have painted once.
     var interval = setInterval(function() {
       if (!p.getPaintCount())
@@ -29,10 +35,6 @@
   }
 
   function doTest() {
-    if (!p.hasWidget()) {
-      todo(false, "This test is only relevant for windowed plugins");
-      return;
-    }
     is(p.getClipRegionRectCount(), 1, "getClipRegionRectCount should be a single rect");
     is(p.getClipRegionRectEdge(0,2) - p.getClipRegionRectEdge(0,0), 100, "width of clip region rect");
     is(p.getClipRegionRectEdge(0,3) - p.getClipRegionRectEdge(0,1), 50, "height of clip region rect");

layout/base/nsDisplayList.cpp

@@ -432,9 +432,12 @@ TreatAsOpaque(nsDisplayItem* aItem, nsDisplayListBuilder* aBuilder,
 {
   nsRegion opaque = aItem->GetOpaqueRegion(aBuilder, aTransparentBackground);
   if (aBuilder->IsForPluginGeometry()) {
-    // Treat all chrome items as opaque
+    // Treat all 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
+    // effectively ignored, as it should be.
     nsIFrame* f = aItem->GetUnderlyingFrame();
-    if (f && f->PresContext()->IsChrome()) {
+    if (f && f->PresContext()->IsChrome() && f->GetStyleDisplay()->mOpacity != 0.0) {
       opaque = aItem->GetBounds(aBuilder);
     }
   }

layout/generic/nsFrame.cpp

@@ -1466,8 +1466,8 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   nsRect absPosClip;
   const nsStyleDisplay* disp = GetStyleDisplay();
   // We can stop right away if this is a zero-opacity stacking context and
-  // we're not checking for event handling.
-  if (disp->mOpacity == 0.0 && !aBuilder->IsForEventDelivery())
+  // we're painting.
+  if (disp->mOpacity == 0.0 && aBuilder->IsForPainting())
     return NS_OK;
 
   PRBool applyAbsPosClipping =