Bug 1536157 - Schedule a paint when setting a pending visual scroll update. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D23901

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

layout/base/PresShell.cpp

@@ -10699,6 +10699,19 @@ bool nsIPresShell::SetVisualViewportOffset(
   return didChange;
 }
 
+void nsIPresShell::SetPendingVisualScrollUpdate(
+    const nsPoint& aVisualViewportOffset,
+    FrameMetrics::ScrollOffsetUpdateType aUpdateType) {
+  mPendingVisualScrollUpdate =
+      mozilla::Some(VisualScrollUpdate{aVisualViewportOffset, aUpdateType});
+
+  // The pending update is picked up during the next paint.
+  // Schedule a paint to make sure one will happen.
+  if (nsIFrame* rootFrame = GetRootFrame()) {
+    rootFrame->SchedulePaint();
+  }
+}
+
 nsPoint nsIPresShell::GetVisualViewportOffsetRelativeToLayoutViewport() const {
   return GetVisualViewportOffset() - GetLayoutViewportOffset();
 }

layout/base/nsIPresShell.h

@@ -1697,10 +1697,7 @@ class nsIPresShell : public nsStubDocumentObserver {
   // Please request APZ review if adding a new call site.
   void SetPendingVisualScrollUpdate(
       const nsPoint& aVisualViewportOffset,
-      FrameMetrics::ScrollOffsetUpdateType aUpdateType) {
-    mPendingVisualScrollUpdate =
-        mozilla::Some(VisualScrollUpdate{aVisualViewportOffset, aUpdateType});
-  }
+      FrameMetrics::ScrollOffsetUpdateType aUpdateType);
   void ClearPendingVisualScrollUpdate() {
     mPendingVisualScrollUpdate = mozilla::Nothing();
   }