I think we should actually do this sooner rather than later.

I have a patch that hacks this in:
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -2663,6 +2663,13 @@ void FrameView::synchronizedPaint()
     ASSERT(!view.isNull());
     forAllNonThrottledFrameViews([](FrameView& frameView) { frameView.lifecycle().advanceTo(DocumentLifecycle::InPaint); });
 
+    PaintLayer* layer = layoutViewItem().layer();
+    if (RuntimeEnabledFeatures::slimmingPaintV2Enabled() && layer && layer->hasCompositedLayerMapping()) {
+        if (GraphicsLayer* mainGraphicsLayer = layer->compositedLayerMapping()->mainGraphicsLayer())
+            synchronizedPaintRecursively(mainGraphicsLayer);
+        if (GraphicsLayer* scrollingContentsLayer = layer->compositedLayerMapping()->scrollingContentsLayer())
+            synchronizedPaintRecursively(scrollingContentsLayer);
+    }
     // A null graphics layer can occur for painting of SVG images that are not parented into the main frame tree,
     // or when the FrameView is the main frame view of a page overlay. The page overlay is in the layer tree of
     // the host page and will be painted during synchronized painting of the host page.
@@ -2719,10 +2726,12 @@ void FrameView::pushPaintArtifactToCompositor()
     ASSERT(layer);
     if (!layer->hasCompositedLayerMapping())
         return;
-    GraphicsLayer* rootGraphicsLayer = layer->compositedLayerMapping()->mainGraphicsLayer();
-    if (!rootGraphicsLayer->drawsContent())
+    GraphicsLayer* contentsLayer = layer->compositedLayerMapping()->scrollingContentsLayer();
+    if (!contentsLayer)
+        contentsLayer = layer->compositedLayerMapping()->mainGraphicsLayer();
+    if (!contentsLayer || !contentsLayer->drawsContent())
         return;
-    const PaintArtifact& paintArtifact = rootGraphicsLayer->getPaintController().paintArtifact();
+    const PaintArtifact& paintArtifact = contentsLayer->getPaintController().paintArtifact();

Presumably this has been fixed.