If a compositing update is declared as needed, scheduleAnimation should be triggered.

Source/core/testing/Internals.cpp

Index: Source/core/testing/Internals.cpp
diff --git a/Source/core/testing/Internals.cpp b/Source/core/testing/Internals.cpp
index eb9c734816a8adb712a37dd162af04ba29dda546..97a38f92758aa65f0ff6f2af3e3ada3f7b3ff5a3 100644
--- a/Source/core/testing/Internals.cpp
+++ b/Source/core/testing/Internals.cpp
@@ -2323,6 +2323,16 @@ void Internals::forceCompositingUpdate(Document* document, ExceptionState& excep
         view->compositor()->updateCompositingLayers();
 }
 
+bool Internals::isCompositorFramePending(Document* document, ExceptionState& exceptionState)
+{
+    if (!document || !document->renderView()) {
+        exceptionState.throwDOMException(InvalidAccessError, document ? "The document's render view cannot be retrieved." : "The document provided is invalid.");
+        return false;
+    }
+
+    return document->page()->chrome().client().isCompositorFramePending();
+}
+
 void Internals::setZoomFactor(float factor)
 {
     frame()->setPageZoomFactor(factor);