Bug 1509575 - Restore the visual viewport offset from the main thread during a first paint. r=kats

During a "first paint" transaction, compositor-side state such as APZ's copy
of the visual viewport offset is overwritten. However, the scroll frame may
persist on the main thread, and in such a case we want to restore the visual
viewport offset stored in the scroll frame. This comes into play during e.g.
navigation back to a page.

Differential Revision: https://phabricator.services.mozilla.com/D16238

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

layout/base/nsLayoutUtils.cpp

@@ -8708,6 +8708,13 @@ static void MaybeReflowForInflationScreenSizeChange(
     metrics.SetBaseScrollOffset(apzScrollPosition);
 
     if (aIsRootContent) {
+      if (aLayerManager->GetIsFirstPaint()) {
+        // Restore the visual viewport offset to the copy stored on the
+        // main thread.
+        presShell->SetPendingVisualViewportOffset(
+            Some(presShell->GetVisualViewportOffset()));
+      }
+
       if (const Maybe<nsPoint>& visualOffset =
               presShell->GetPendingVisualViewportOffset()) {
         metrics.SetVisualViewportOffset(CSSPoint::FromAppUnits(*visualOffset));