Bug 1664626 - Move one of the conditions for ignoring a visual scroll update. r=botond

This should be equivalent but a little easier to reason about.

Differential Revision: https://phabricator.services.mozilla.com/D90721

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4514,18 +4514,9 @@ void AsyncPanZoomController::NotifyLayersUpdated(
   // particular location. This is different from a layout viewport offset update
   // in that the layout viewport offset is limited to the layout scroll range,
   // while the visual viewport offset is not.
-  // The update type indicates the priority; an eMainThread layout update (or
-  // a smooth scroll request which is similar) takes precedence over an eRestore
-  // visual update, but we allow the possibility for the main thread to ask us
-  // to scroll both the layout and visual viewports to distinct (but compatible)
-  // locations (via e.g. both updates being eRestore).
   bool visualScrollOffsetUpdated =
       isDefault ||
       aLayerMetrics.GetVisualScrollUpdateType() != FrameMetrics::eNone;
-  if (aLayerMetrics.GetScrollUpdateType() == FrameMetrics::eMainThread &&
-      aLayerMetrics.GetVisualScrollUpdateType() != FrameMetrics::eMainThread) {
-    visualScrollOffsetUpdated = false;
-  }
 
   // TODO if we're in a drag and scrollOffsetUpdated is set then we want to
   // ignore it
@@ -4727,6 +4718,20 @@ void AsyncPanZoomController::NotifyLayersUpdated(
     MOZ_ASSERT(scrollUpdate.GetMode() == ScrollMode::Instant ||
                scrollUpdate.GetMode() == ScrollMode::Normal);
 
+    // If the layout update is of a higher priority than the visual update, then
+    // we don't want to apply the visual update.
+    // If the layout update is of a clobbering type (or a smooth scroll request,
+    // which is handled above) then it takes precedence over an eRestore visual
+    // update. But we also allow the possibility for the main thread to ask us
+    // to scroll both the layout and visual viewports to distinct (but
+    // compatible) locations (via e.g. both updates being of a non-clobbering/
+    // eRestore type).
+    if (nsLayoutUtils::CanScrollOriginClobberApz(scrollUpdate.GetOrigin()) &&
+        aLayerMetrics.GetVisualScrollUpdateType() !=
+            FrameMetrics::eMainThread) {
+      visualScrollOffsetUpdated = false;
+    }
+
     Maybe<CSSPoint> relativeDelta;
 
     if (StaticPrefs::apz_relative_update_enabled() &&