From 0fd2537d8e1cdbd382739a61caf733d5a4ae3d19 Mon Sep 17 00:00:00 2001
From: Timothy Nikkel <tnikkel@gmail.com>
Date: Sun, 13 Sep 2020 15:27:34 +0000
Subject: [PATCH] Bug 1664661. Make ScrollFrameHelper::UpdateScrollbarPosition
 use the same calculation for scrollbar attributes as
 ScrollFrameHelper::ReflowFinished. r=kats

UpdateScrollbarPosition should have been using scroll range and not scrolled rect for a long time. It didn't matter because we only used the top left of the rect which matched until bug 1654933 made GetScrollRangeForUserInputEvents return a rect that wasn't located at 0,0.

Differential Revision: https://phabricator.services.mozilla.com/D90031
---
 layout/generic/nsGfxScrollFrame.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index 00faa98e94a3..c6eca46c581d 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -5371,22 +5371,23 @@ void ScrollFrameHelper::UpdateScrollbarPosition() {
   mFrameIsUpdatingScrollbar = true;
 
   nsPoint pt = GetScrollPosition();
-  nsRect scrolledRect = GetScrolledRect();
+  nsRect scrollRange = GetVisualScrollRange();
 
   if (gfxPlatform::UseDesktopZoomingScrollbars()) {
     pt = GetVisualViewportOffset();
+    scrollRange = GetScrollRangeForUserInputEvents();
   }
 
   if (mVScrollbarBox) {
     SetCoordAttribute(mVScrollbarBox->GetContent()->AsElement(),
-                      nsGkAtoms::curpos, pt.y - scrolledRect.y);
+                      nsGkAtoms::curpos, pt.y - scrollRange.y);
     if (!weakFrame.IsAlive()) {
       return;
     }
   }
   if (mHScrollbarBox) {
     SetCoordAttribute(mHScrollbarBox->GetContent()->AsElement(),
-                      nsGkAtoms::curpos, pt.x - scrolledRect.x);
+                      nsGkAtoms::curpos, pt.x - scrollRange.x);
     if (!weakFrame.IsAlive()) {
       return;
     }