From 77b4f9fdf9d97228ce3f1d824aea62632d2e7616 Mon Sep 17 00:00:00 2001
From: Kartikaya Gupta <kgupta@mozilla.com>
Date: Fri, 3 Jul 2020 15:55:56 +0000
Subject: [PATCH] Bug 1644567 - Use a more appropriate check when reading the
 visual viewport offset. r=botond

Instead of checking for the visual viewport *size* being set, we check for
the *offset* being set. This makes more sense becuase we're going to be
reading the offset, not the size.

Depends on D81740

Differential Revision: https://phabricator.services.mozilla.com/D81741
---
 layout/generic/nsGfxScrollFrame.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index fc83979bae5c..ff705234c5e5 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -4283,11 +4283,13 @@ nsSize ScrollFrameHelper::GetVisualViewportSize() const {
 
 nsPoint ScrollFrameHelper::GetVisualViewportOffset() const {
   PresShell* presShell = mOuter->PresShell();
-  if (mIsRoot && presShell->IsVisualViewportSizeSet()) {
+  if (mIsRoot) {
     if (auto pendingUpdate = presShell->GetPendingVisualScrollUpdate()) {
       return pendingUpdate->mVisualScrollOffset;
     }
-    return presShell->GetVisualViewportOffset();
+    if (presShell->IsVisualViewportOffsetSet()) {
+      return presShell->GetVisualViewportOffset();
+    }
   }
   return GetScrollPosition();
 }