Bug 1031107 - Part 6: Subtract from the visible region in nsDisplaySc…

…rollLayer if we're not using APZ. r=tn

layout/base/nsDisplayList.cpp

@@ -3603,9 +3603,20 @@ nsDisplaySubDocument::ComputeVisibility(nsDisplayListBuilder* aBuilder,
   bool visible = mList.ComputeVisibilityForSublist(
     aBuilder, &childVisibleRegion, boundedRect,
     usingDisplayPort ? mFrame : nullptr);
-  // We don't allow this computation to influence aVisibleRegion, on the
-  // assumption that the layer can be asynchronously scrolled so we'll
-  // definitely need all the content under it.
+
+#ifndef MOZ_WIDGET_ANDROID
+  // If APZ is enabled then don't allow this computation to influence
+  // aVisibleRegion, on the assumption that the layer can be asynchronously
+  // scrolled so we'll definitely need all the content under it.
+  if (!gfxPrefs::AsyncPanZoomEnabled()) {
+    bool snap;
+    nsRect bounds = GetBounds(aBuilder, &snap);
+    nsRegion removed;
+    removed.Sub(bounds, childVisibleRegion);
+
+    aBuilder->SubtractFromVisibleRegion(aVisibleRegion, removed);
+  }
+#endif
 
   return visible;
 }
@@ -3904,9 +3915,19 @@ nsDisplayScrollLayer::ComputeVisibility(nsDisplayListBuilder* aBuilder,
   bool visible = mList.ComputeVisibilityForSublist(
     aBuilder, &childVisibleRegion, boundedRect,
     usingDisplayPort ? mScrollFrame : nullptr);
-  // We don't allow this computation to influence aVisibleRegion, on the
-  // assumption that the layer can be asynchronously scrolled so we'll
-  // definitely need all the content under it.
+
+#ifndef MOZ_WIDGET_ANDROID
+  // If APZ is enabled then don't allow this computation to influence
+  // aVisibleRegion, on the assumption that the layer can be asynchronously
+  // scrolled so we'll definitely need all the content under it.
+  if (!gfxPrefs::AsyncPanZoomEnabled()) {
+    bool snap;
+    nsRect bounds = GetBounds(aBuilder, &snap);
+    nsRegion removed;
+    removed.Sub(bounds, childVisibleRegion);
+    aBuilder->SubtractFromVisibleRegion(aVisibleRegion, removed);
+  }
+#endif
 
   return visible;
 }