Only automatically promote scrollers which are untransformed and opaque.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
@@ skipping 1666 matching lines @@
   if (!layer->scrollsOverflow())
     return false;
 
   Node* node = layer->enclosingNode();
   if (node && node->isElementNode() &&
       (toElement(node)->compositorMutableProperties() &
        (CompositorMutableProperty::kScrollTop |
         CompositorMutableProperty::kScrollLeft)))
     return true;
 
-  // TODO(schenney): LCD Text also requires integer scroll offsets for the
-  // layer. While we use integer scroll offsets locally when
-  // !layer->compositor()->preferCompositingToLCDTextEnabled(), we do not check
-  // offsets accumulated from the root (including translates).  crbug.com/644833
+  // TODO(flackr): Allow integer transforms as long as all of the ancestor
+  // transforms are also integer.
   bool backgroundSupportsLCDText =
       RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled() &&
       layer->canPaintBackgroundOntoScrollingContentsLayer() &&
       layer->backgroundIsKnownToBeOpaqueInRect(
-          toLayoutBox(layer->layoutObject())->paddingBoxRect());
+          toLayoutBox(layer->layoutObject())->paddingBoxRect()) &&
+      !layer->transformAncestor() && !layer->transform() &&
+      !layer->opacityAncestor() &&
+      !layer->layoutObject()->style()->hasOpacity();
   if (mode == PaintLayerScrollableArea::ConsiderLCDText &&
       !layer->compositor()->preferCompositingToLCDTextEnabled() &&
       !backgroundSupportsLCDText)
     return false;
 
   // TODO(schenney) Tests fail if we do not also exclude
   // layer->layoutObject()->style()->hasBorderDecoration() (missing background
   // behind dashed borders). Resolve this case, or not, and update this check
   // with the results.
   return !(layer->size().isEmpty() || layer->hasDescendantWithClipPath() ||
@@ skipping 273 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetsAfterLayout();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink