Ignore should-scroll-on-main-thread if main frame is not scrollable

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index e9f01bf4c49ee77a7c32a8a8b36e23da1fe12952..4ae90e603cab9f80199ab44e5d1306a2720a44e0 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -791,8 +791,11 @@ bool ScrollingCoordinator::hasVisibleSlowRepaintViewportConstrainedObjects(Frame
 
 MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() const
 {
+    // The main thread scrolling reasons are applicable to scrolls of the main
+    // frame. If it does not exist or if it is not scrollable, there is no
+    // reason to force main thread scrolling.
     FrameView* frameView = m_page->mainFrame()->view();
-    if (!frameView)
+    if (!frameView || !frameView->isScrollable())
         return static_cast<MainThreadScrollingReasons>(0);
 
     MainThreadScrollingReasons mainThreadScrollingReasons = (MainThreadScrollingReasons)0;