Need to repaint its ::first-line background when a block moves.

third_party/WebKit/Source/core/layout/LayoutBlockFlowLine.cpp

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All right reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 2061 matching lines @@
     return logicalLeft;
 }
 
 void LayoutBlockFlow::invalidateDisplayItemClientsOfFirstLine()
 {
     ASSERT(childrenInline());
     if (RootInlineBox* firstRootBox = this->firstRootBox())
         firstRootBox->invalidateDisplayItemClientsRecursively();
 }
 
+PaintInvalidationReason LayoutBlockFlow::invalidatePaintIfNeeded(PaintInvalidationState& paintInvalidationState, const LayoutBoxModelObject& paintInvalidationContainer)
+{
+    PaintInvalidationReason reason = LayoutBlock::invalidatePaintIfNeeded(paintInvalidationState, paintInvalidationContainer);
+    if (reason == PaintInvalidationNone)
+        return reason;
+    RootInlineBox* line = firstRootBox();
+    if (!line || !line->isFirstLineStyle())
+        return reason;
+    // It's the RootInlineBox that paints the ::first-line background. Note that since it may be
+    // expensive to figure out if the first line is affected by any ::first-line selectors at all,
+    // we just invalidate it unconditionally, since that's typically cheaper.
+    invalidateDisplayItemClient(*line);
+    return reason;
 }
+
+}