Bug 1611660 - Accept layout viewport updates from the main thread right away. r=tnikkel

Previously, we would wait until the following frame (for uncertain reasons
that date back to B2G), but this meant the layout and visual viewports would
be out of sync for a frame, causing APZ to misbehave.

Differential Revision: https://phabricator.services.mozilla.com/D61286

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

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4548,28 +4548,13 @@ void AsyncPanZoomController::NotifyLayersUpdated(
   RepaintUpdateType contentRepaintType = RepaintUpdateType::eNone;
   bool viewportUpdated = false;
 
-  // We usually don't entertain viewport updates on the same transaction as
-  // a composition bounds update, but we make an exception for Android
-  // to avoid the composition bounds and the viewport diverging during
-  // orientation changes and dynamic toolbar transitions.
-  // TODO: Do this on all platforms.
-  bool entertainViewportUpdates =
-      FuzzyEqualsAdditive(aLayerMetrics.GetCompositionBounds().Width(),
-                          Metrics().GetCompositionBounds().Width()) &&
-      FuzzyEqualsAdditive(aLayerMetrics.GetCompositionBounds().Height(),
-                          Metrics().GetCompositionBounds().Height());
-#if defined(MOZ_WIDGET_ANDROID)
-  entertainViewportUpdates = true;
-#endif
-  if (entertainViewportUpdates) {
-    if (Metrics().GetLayoutViewport().Size() !=
-        aLayerMetrics.GetLayoutViewport().Size()) {
-      needContentRepaint = true;
-      viewportUpdated = true;
-    }
-    if (viewportUpdated || scrollOffsetUpdated) {
-      Metrics().SetLayoutViewport(aLayerMetrics.GetLayoutViewport());
-    }
+  if (Metrics().GetLayoutViewport().Size() !=
+      aLayerMetrics.GetLayoutViewport().Size()) {
+    needContentRepaint = true;
+    viewportUpdated = true;
+  }
+  if (viewportUpdated || scrollOffsetUpdated) {
+    Metrics().SetLayoutViewport(aLayerMetrics.GetLayoutViewport());
   }
 
 #if defined(MOZ_WIDGET_ANDROID)