Bug 1753881. If the visual viewport offset hasn't been set on the presshell return the layout scroll position. r=botond

Because that is the correct value.

Differential Revision: https://phabricator.services.mozilla.com/D139472

layout/base/PresShell.cpp

@@ -11197,7 +11197,14 @@ bool PresShell::SetVisualViewportOffset(const nsPoint& aScrollOffset,
     }
   }
 
-  nsPoint prevOffset = GetVisualViewportOffset();
+  // Careful here not to call GetVisualViewportOffset to get the previous visual
+  // viewport offset because if mVisualViewportOffset is nothing then we'll get
+  // the layout scroll position directly from the scroll frame and it has likely
+  // already been updated.
+  nsPoint prevOffset = aPrevLayoutScrollPos;
+  if (mVisualViewportOffset.isSome()) {
+    prevOffset = *mVisualViewportOffset;
+  }
   if (prevOffset == newOffset) {
     return false;
   }

layout/base/PresShell.h

@@ -1481,7 +1481,10 @@ class PresShell final : public nsStubDocumentObserver,
                                const nsPoint& aPrevLayoutScrollPos);
 
   nsPoint GetVisualViewportOffset() const {
-    return mVisualViewportOffset.valueOr(nsPoint());
+    if (mVisualViewportOffset.isSome()) {
+      return *mVisualViewportOffset;
+    }
+    return GetLayoutViewportOffset();
   }
   bool IsVisualViewportOffsetSet() const {
     return mVisualViewportOffset.isSome();