Bug 1421071 - Don't use fallback for invisible overlay scrollbar tracks on Mac with WebRender. r=jrmuizel

MozReview-Commit-ID: 5PJVqgGi8uD

--HG--
extra : rebase_source : bc238065ee46d009120dde7ee8de56f640facb24

widget/cocoa/nsNativeThemeCocoa.mm

@@ -3080,10 +3080,19 @@ nsNativeThemeCocoa::CreateWebRenderCommandsForWidget(mozilla::wr::DisplayListBui
     case NS_THEME_RANGE:
     case NS_THEME_SCROLLBARTHUMB_VERTICAL:
     case NS_THEME_SCROLLBARTHUMB_HORIZONTAL:
-    case NS_THEME_SCROLLBARTRACK_HORIZONTAL:
-    case NS_THEME_SCROLLBARTRACK_VERTICAL:
       return false;
 
+    case NS_THEME_SCROLLBARTRACK_HORIZONTAL:
+    case NS_THEME_SCROLLBARTRACK_VERTICAL: {
+      BOOL isOverlay = nsLookAndFeel::UseOverlayScrollbars();
+      if (isOverlay && !IsParentScrollbarRolledOver(aFrame)) {
+        // There is no scrollbar track, draw nothing and return true.
+        return true;
+      }
+      // There is a scrollbar track and it needs to be drawn using fallback.
+      return false;
+    }
+
     case NS_THEME_TEXTFIELD_MULTILINE: {
       if (eventState.HasState(NS_EVENT_STATE_FOCUS)) {
         // We can't draw the focus ring using webrender, so fall back to regular