From 20a0c0f910470656795150af06371d7be1587145 Mon Sep 17 00:00:00 2001
From: Markus Stange <mstange@themasta.com>
Date: Mon, 27 Nov 2017 17:35:36 -0500
Subject: [PATCH] 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 | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/widget/cocoa/nsNativeThemeCocoa.mm b/widget/cocoa/nsNativeThemeCocoa.mm
index 927cd7176f91..760279b3a972 100644
--- a/widget/cocoa/nsNativeThemeCocoa.mm
+++ b/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