Bug 1915051 - Add an assertion to ensure hit testing and rendering are not out of sync. r=hiro,tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D220201

Author: theres-waldo

layout/painting/nsDisplayList.cpp

@@ -847,6 +847,17 @@ void nsDisplayListBuilder::UpdateShouldBuildAsyncZoomContainer() {
   mBuildAsyncZoomContainer = !mIsRelativeToLayoutViewport &&
                              !document->Fullscreen() &&
                              nsLayoutUtils::AllowZoomingForDocument(document);
+
+  // If mIsRelativeToLayoutViewport == false, hit-testing on this
+  // display list will take into account the pres shell resolution.
+  // If we're not building an async zoom container (meaning, the
+  // resolution will not take effect visually), the resolution better
+  // be 1.0, otherwise rendering and hit-testing are out of sync.
+#ifdef DEBUG
+  if (!mIsRelativeToLayoutViewport && !mBuildAsyncZoomContainer) {
+    MOZ_ASSERT(document->GetPresShell()->GetResolution() == 1.0f);
+  }
+#endif
 }
 
 // Certain prefs may cause display list items to be added or removed when they