Bug 1871751. Avoid AsyncPanZoomEnabled call in MaybeCreateDisplayPortInFirstScrollFrameEncountered. r=layout-reviewers,emilio

The return value of nsLayoutUtils::AsyncPanZoomEnabled only changes if we descend into a frame whose display root widget is different. But that is not possible, we do not descend into any frames that are in different widgets.

Differential Revision: https://phabricator.services.mozilla.com/D197237

layout/base/DisplayPortUtils.cpp

@@ -790,8 +790,8 @@ bool DisplayPortUtils::MaybeCreateDisplayPort(
   // async-scrollable frame (i.e. one that WantsAsyncScroll()) has a
   // displayport. If that's not the case yet, and we are async-scrollable, we
   // will get a displayport.
+  MOZ_ASSERT(nsLayoutUtils::AsyncPanZoomEnabled(aScrollFrame));
   if (!aBuilder->HaveScrollableDisplayPort() &&
-      nsLayoutUtils::AsyncPanZoomEnabled(aScrollFrame) &&
       aScrollFrameAsScrollable->WantAsyncScroll()) {
     bool haveDisplayPort = HasNonMinimalNonZeroDisplayPort(content);
     // If we don't already have a displayport, calculate and set one.

layout/base/nsLayoutUtils.cpp

@@ -3168,7 +3168,8 @@ void nsLayoutUtils::PaintFrame(gfxContext* aRenderingContext, nsIFrame* aFrame,
   }
 
   builder->ClearHaveScrollableDisplayPort();
-  if (builder->IsPaintingToWindow()) {
+  if (builder->IsPaintingToWindow() &&
+      nsLayoutUtils::AsyncPanZoomEnabled(aFrame)) {
     DisplayPortUtils::MaybeCreateDisplayPortInFirstScrollFrameEncountered(
         aFrame, builder);
   }