Non-composited, fixed position should trigger main thread scrolling

Source/core/rendering/compositing/CompositingReasonFinder.cpp

Index: Source/core/rendering/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/rendering/compositing/CompositingReasonFinder.cpp b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
index 61451afd06f3e8c32ca10be225f5526102a0d41a..6f6ce3d3ca2cfbe9708b2d3820d7b70576ed9138 100644
--- a/Source/core/rendering/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
@@ -149,7 +149,7 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
             directReasons |= CompositingReasonOverflowScrollingTouch;
     }
 
-    if (requiresCompositingForPositionFixed(renderer))
+    if (requiresCompositingForPositionFixed(layer))
         directReasons |= CompositingReasonPositionFixed;
 
     directReasons |= renderer->additionalCompositingReasons();
@@ -166,15 +166,13 @@ bool CompositingReasonFinder::requiresCompositingForAnimation(RenderStyle* style
     return style->shouldCompositeForCurrentAnimations();
 }
 
-bool CompositingReasonFinder::requiresCompositingForPositionFixed(RenderObject* renderer) const
+bool CompositingReasonFinder::requiresCompositingForPositionFixed(const RenderLayer* layer) const
 {
     if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger))
         return false;
     // Don't promote fixed position elements that are descendants of a non-view container, e.g. transformed elements.
     // They will stay fixed wrt the container rather than the enclosing frame.
-    return renderer->style()->position() == FixedPosition
-        && renderer->container() == &m_renderView
-        && m_renderView.frameView()->isScrollable();
+    return layer->scrollsWithViewport() && m_renderView.frameView()->isScrollable();
 }
 
 }