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

@@ -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();
 }