Bug 1605956 - Don't invalidate scroll anchor on visual viewport offset updates if visual viewport size is not set. r=botond

As we only use the offset if the visual viewport size is set.

Differential Revision: https://phabricator.services.mozilla.com/D58226

--HG--
extra : moz-landing-system : lando

layout/base/PresShell.cpp

@@ -10649,24 +10649,25 @@ void PresShell::ResetVisualViewportSize() {
 
 bool PresShell::SetVisualViewportOffset(const nsPoint& aScrollOffset,
                                         const nsPoint& aPrevLayoutScrollPos) {
-  bool didChange = false;
-  if (GetVisualViewportOffset() != aScrollOffset) {
-    nsPoint prevOffset = GetVisualViewportOffset();
-    mVisualViewportOffset = Some(aScrollOffset);
-    didChange = true;
+  nsPoint prevOffset = GetVisualViewportOffset();
+  if (prevOffset == aScrollOffset) {
+    return false;
+  }
 
-    if (auto* window = nsGlobalWindowInner::Cast(mDocument->GetInnerWindow())) {
-      window->VisualViewport()->PostScrollEvent(prevOffset,
-                                                aPrevLayoutScrollPos);
-    }
+  mVisualViewportOffset = Some(aScrollOffset);
 
+  if (auto* window = nsGlobalWindowInner::Cast(mDocument->GetInnerWindow())) {
+    window->VisualViewport()->PostScrollEvent(prevOffset, aPrevLayoutScrollPos);
+  }
+
+  if (IsVisualViewportSizeSet()) {
     if (nsIScrollableFrame* rootScrollFrame =
             GetRootScrollFrameAsScrollable()) {
-      ScrollAnchorContainer* container = rootScrollFrame->Anchor();
-      container->UserScrolled();
+      rootScrollFrame->Anchor()->UserScrolled();
     }
   }
-  return didChange;
+
+  return true;
 }
 
 void PresShell::ScrollToVisual(const nsPoint& aVisualViewportOffset,