Bug 1531796 - Call NotifyApzTransaction() for skipped paints, too. r=rhunt,kats

We need to do this separately in the WR and non-WR codepaths for picking up
scroll updates for skipped paints.

Differential Revision: https://phabricator.services.mozilla.com/D29061

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

gfx/layers/Layers.cpp

@@ -616,10 +616,12 @@ void Layer::ApplyPendingUpdatesForThisTransaction() {
 
   for (size_t i = 0; i < mScrollMetadata.Length(); i++) {
     FrameMetrics& fm = mScrollMetadata[i].GetMetrics();
+    ScrollableLayerGuid::ViewID scrollId = fm.GetScrollId();
     Maybe<ScrollUpdateInfo> update =
-        Manager()->GetPendingScrollInfoUpdate(fm.GetScrollId());
+        Manager()->GetPendingScrollInfoUpdate(scrollId);
     if (update) {
       fm.UpdatePendingScrollInfo(update.value());
+      nsLayoutUtils::NotifyPaintSkipTransaction(scrollId);
       Mutated();
     }
   }

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -248,6 +248,9 @@ bool WebRenderLayerManager::EndEmptyTransaction(EndTransactionFlags aFlags) {
                                                   updates->mLargeShmems);
       }
       updates->mScrollUpdates = std::move(mPendingScrollUpdates[renderRoot]);
+      for (const auto& entry : updates->mScrollUpdates) {
+        nsLayoutUtils::NotifyPaintSkipTransaction(/*scroll id=*/entry.first);
+      }
     }
   }
 

layout/base/nsLayoutUtils.cpp

@@ -1388,6 +1388,13 @@ void nsLayoutUtils::RemoveDisplayPort(nsIContent* aContent) {
   aContent->DeleteProperty(nsGkAtoms::DisplayPortMargins);
 }
 
+void nsLayoutUtils::NotifyPaintSkipTransaction(ViewID aScrollId) {
+  if (nsIScrollableFrame* scrollFrame =
+          nsLayoutUtils::FindScrollableFrameFor(aScrollId)) {
+    scrollFrame->NotifyApzTransaction();
+  }
+}
+
 nsContainerFrame* nsLayoutUtils::LastContinuationWithChild(
     nsContainerFrame* aFrame) {
   MOZ_ASSERT(aFrame, "NULL frame pointer");

layout/base/nsLayoutUtils.h

@@ -321,6 +321,18 @@ class nsLayoutUtils {
    */
   static void RemoveDisplayPort(nsIContent* aContent);
 
+  /**
+   * Notify the scroll frame with the given scroll id that its scroll offset
+   * is being sent to APZ as part of a paint-skip transaction.
+   *
+   * Normally, this notification happens during painting, after calls to
+   * ComputeScrollMetadata(). During paint-skipping that code is skipped,
+   * but it's still important for the scroll frame to be notified for
+   * correctness of relative scroll updates, so the code that sends the
+   * empty paint-skip transaction needs to call this.
+   */
+  static void NotifyPaintSkipTransaction(ViewID aScrollId);
+
   /**
    * Use heuristics to figure out the child list that
    * aChildFrame is currently in.