Don't rely on values computed as a side effect of paint.

Source/core/rendering/RenderLayerScrollableArea.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@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 377 matching lines @@
     if (repaintContainer)
         quadForFakeMouseMoveEvent = repaintContainer->localToAbsoluteQuad(quadForFakeMouseMoveEvent);
     frame->eventHandler().dispatchFakeMouseMoveEventSoonInQuad(quadForFakeMouseMoveEvent);
 
     bool requiresRepaint = true;
 
     if (m_box->view()->compositor()->inCompositingMode()) {
         bool onlyScrolledCompositedLayers = scrollsOverflow()
             && !layer()->hasVisibleNonLayerContent()
             && !layer()->hasNonCompositedChild()
-            && !layer()->hasBlockSelectionGapBounds()
-            && !m_box->isMarquee();
+            && !m_box->isMarquee()
+            && layer()->renderer()->selectionRectForRepaint(0, true).isEmpty();

[abarth-chromium, 2014/03/21 15:45:54]

Why not just call selectionRect() ?  That supplies these values for you.

[Ian Vollick, 2014/03/21 17:01:50]

Thanks, that's way better. Done.

 
         if (usesCompositedScrolling() || onlyScrolledCompositedLayers)
             requiresRepaint = false;
     }
 
     // Just schedule a full repaint of our object.
     if (requiresRepaint) {
         if (RuntimeEnabledFeatures::repaintAfterLayoutEnabled()) {
             if (m_box->frameView()->isInPerformLayout())
                 m_box->setShouldDoFullRepaintAfterLayout(true);
@@ skipping 1135 matching lines @@
 void RenderLayerScrollableArea::setForceNeedsCompositedScrolling(ForceNeedsCompositedScrollingMode mode)
 {
     if (m_forceNeedsCompositedScrolling == mode)
         return;
 
     m_forceNeedsCompositedScrolling = mode;
     layer()->didUpdateNeedsCompositedScrolling();
 }
 
 } // Namespace WebCore