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

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

Index: third_party/WebKit/Source/core/layout/LayoutBlockFlowLine.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlockFlowLine.cpp b/third_party/WebKit/Source/core/layout/LayoutBlockFlowLine.cpp
index 70afd80a4198c4f2b5a63388252fabe4d272baf3..19518e3248b9a1a6fa8b129b10063c72fa4a9de6 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlockFlowLine.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBlockFlowLine.cpp
@@ -2079,4 +2079,19 @@ void LayoutBlockFlow::invalidateDisplayItemClientsOfFirstLine()
         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;
+}
+
 }