Bug 1548247 - Correct and limit scroll update renderroot usage r=kats

There's two things going on here. 1) nsGfxScrollFrame is getting the
wrong renderroot, because it's not correctly recursing up the frame
tree. 2) Hiding behind that problem is that if we do correctly assign
the renderroot, we end up blocking on both render roots updating if
we don't, say, have a horizontal scroll option, because that leaves
us with a wr::RenderRoot::Default. 2.1) We then still end up blocking
on the default renderroot because we initialize the selector with
WebRenderBridgeParent's mRenderRoot.

Differential Revision: https://phabricator.services.mozilla.com/D31858

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

gfx/layers/apz/src/APZUpdater.cpp

@@ -157,12 +157,18 @@ void APZUpdater::UpdateFocusState(LayersId aRootLayerTreeId,
                                   WRRootId aOriginatingWrRootId,
                                   const FocusTarget& aFocusTarget) {
   MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
-  UpdaterQueueSelector selector(aOriginatingWrRootId);
+  UpdaterQueueSelector selector(aOriginatingWrRootId.mLayersId);
   if (aFocusTarget.mData.is<FocusTarget::ScrollTargets>()) {
     const FocusTarget::ScrollTargets& targets =
         aFocusTarget.mData.as<FocusTarget::ScrollTargets>();
-    selector.mRenderRoots += targets.mHorizontalRenderRoot;
-    selector.mRenderRoots += targets.mVerticalRenderRoot;
+    if (targets.mHorizontalRenderRoot) {
+      selector.mRenderRoots += *targets.mHorizontalRenderRoot;
+    }
+    if (targets.mVerticalRenderRoot) {
+      selector.mRenderRoots += *targets.mVerticalRenderRoot;
+    }
+  } else {
+    selector.mRenderRoots += aOriginatingWrRootId.mRenderRoot;
   }
   RunOnUpdaterThread(selector,
                      NewRunnableMethod<LayersId, LayersId, FocusTarget>(

gfx/layers/apz/src/FocusTarget.cpp

@@ -207,17 +207,19 @@ FocusTarget::FocusTarget(PresShell* aRootPresShell,
   target.mHorizontal = nsLayoutUtils::FindIDForScrollableFrame(horizontal);
   target.mVertical = nsLayoutUtils::FindIDForScrollableFrame(vertical);
   if (XRE_IsContentProcess()) {
-    target.mHorizontalRenderRoot = gfxUtils::GetContentRenderRoot();
-    target.mVerticalRenderRoot = gfxUtils::GetContentRenderRoot();
+    target.mHorizontalRenderRoot = Some(gfxUtils::GetContentRenderRoot());
+    target.mVerticalRenderRoot = Some(gfxUtils::GetContentRenderRoot());
   } else {
-    target.mHorizontalRenderRoot =
-        horizontal ? gfxUtils::RecursivelyGetRenderRootForFrame(
-                         horizontal->GetScrolledFrame())
-                   : wr::RenderRoot::Default;
-    target.mVerticalRenderRoot =
-        vertical ? gfxUtils::RecursivelyGetRenderRootForFrame(
-                       vertical->GetScrolledFrame())
-                 : wr::RenderRoot::Default;
+    if (horizontal) {
+      auto renderRoot = gfxUtils::RecursivelyGetRenderRootForFrame(
+          horizontal->GetScrolledFrame());
+      target.mHorizontalRenderRoot = Some(renderRoot);
+    }
+    if (vertical) {
+      auto renderRoot = gfxUtils::RecursivelyGetRenderRootForFrame(
+          vertical->GetScrolledFrame());
+      target.mVerticalRenderRoot = Some(renderRoot);
+    }
   }
   mData = AsVariant(target);
 

gfx/layers/apz/src/FocusTarget.h

@@ -13,6 +13,7 @@
 #include "mozilla/layers/ScrollableLayerGuid.h"  // for ViewID
 #include "mozilla/webrender/WebRenderTypes.h"    // for RenderRoot
 #include "mozilla/Variant.h"                     // for Variant
+#include "mozilla/Maybe.h"                       // for Maybe
 
 namespace mozilla {
 
@@ -30,9 +31,9 @@ class FocusTarget final {
  public:
   struct ScrollTargets {
     ScrollableLayerGuid::ViewID mHorizontal;
-    wr::RenderRoot mHorizontalRenderRoot;
+    Maybe<wr::RenderRoot> mHorizontalRenderRoot;
     ScrollableLayerGuid::ViewID mVertical;
-    wr::RenderRoot mVerticalRenderRoot;
+    Maybe<wr::RenderRoot> mVerticalRenderRoot;
 
     bool operator==(const ScrollTargets& aRhs) const {
       bool ret =

gfx/thebes/gfxUtils.cpp

@@ -1511,7 +1511,7 @@ wr::RenderRoot gfxUtils::RecursivelyGetRenderRootForFrame(
   }
 
   for (const nsIFrame* current = aFrame; current;
-       current = current->GetParent()) {
+       current = nsLayoutUtils::GetCrossDocParentFrame(current)) {
     auto renderRoot = gfxUtils::GetRenderRootForFrame(current);
     if (renderRoot) {
       return *renderRoot;

layout/generic/nsGfxScrollFrame.cpp

@@ -2856,9 +2856,7 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
             // might still get squashed into a full transaction if something
             // happens to trigger one.
             wr::RenderRoot renderRoot = wr::RenderRoot::Default;
-            if (XRE_IsContentProcess()) {
-              renderRoot = gfxUtils::GetContentRenderRoot();
-            } else {
+            if (XRE_IsParentProcess()) {
               renderRoot = gfxUtils::RecursivelyGetRenderRootForFrame(mOuter);
             }
             success = manager->SetPendingScrollUpdateForNextTransaction(