Make ScrollbarThemeAura selectively invalidate scrollbar parts.

third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index a86da068c7f3476ad441ca7d09b13d5428fbc8d4..4b6a34163d9ce2b2565215fe0efc98e8dd0c64f7 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -126,19 +126,27 @@ bool Scrollbar::isLeftSideVerticalScrollbar() const
     return false;
 }
 
-void Scrollbar::offsetDidChange()
+bool Scrollbar::offsetDidChange()
 {
     ASSERT(m_scrollableArea);
 
     float position = scrollableAreaCurrentPos();
     if (position == m_currentPos)
-        return;
+        return false;
 
+    float oldPosition = m_currentPos;
     int oldThumbPosition = theme().thumbPosition(*this);
     m_currentPos = position;
-    updateThumbPosition();
+
+    ScrollbarPart invalidParts = theme().invalidateOnThumbPositionChange(
+        *this, oldPosition, position);
+    if (invalidParts != NoPart)
+        setNeedsPaintInvalidation(invalidParts);
+
     if (m_pressedPart == ThumbPart)
         setPressedPos(m_pressedPos + theme().thumbPosition(*this) - oldThumbPosition);
+
+    return true;
 }
 
 void Scrollbar::disconnectFromScrollableArea()
@@ -154,24 +162,9 @@ void Scrollbar::setProportion(int visibleSize, int totalSize)
     m_visibleSize = visibleSize;
     m_totalSize = totalSize;
 
-    updateThumbProportion();
-}
-
-void Scrollbar::updateThumb()
-{
     setNeedsPaintInvalidation();
 }
 
-void Scrollbar::updateThumbPosition()
-{
-    updateThumb();
-}
-
-void Scrollbar::updateThumbProportion()
-{
-    updateThumb();
-}
-
 void Scrollbar::paint(GraphicsContext& context, const CullRect& cullRect) const
 {
     if (!cullRect.intersectsCullRect(frameRect()))
@@ -317,7 +310,7 @@ void Scrollbar::setHoveredPart(ScrollbarPart part)
     if (((m_hoveredPart == NoPart || part == NoPart) && theme().invalidateOnMouseEnterExit())
         // When there's a pressed part, we don't draw a hovered state, so there's no reason to invalidate.
         || m_pressedPart == NoPart)
-        setNeedsPaintInvalidation();
+        setNeedsPaintInvalidation(static_cast<ScrollbarPart>(m_hoveredPart | part));
 
     m_hoveredPart = part;
 }
@@ -327,7 +320,7 @@ void Scrollbar::setPressedPart(ScrollbarPart part)
     if (m_pressedPart != NoPart
         // When we no longer have a pressed part, we can start drawing a hovered state on the hovered part.
         || m_hoveredPart != NoPart)
-        setNeedsPaintInvalidation();
+        setNeedsPaintInvalidation(static_cast<ScrollbarPart>(m_pressedPart | m_hoveredPart | part));
     m_pressedPart = part;
 }
 
@@ -562,12 +555,14 @@ float Scrollbar::scrollableAreaCurrentPos() const
     return m_scrollableArea->scrollPosition().y() - m_scrollableArea->minimumScrollPosition().y();
 }
 
-void Scrollbar::setNeedsPaintInvalidation()
+void Scrollbar::setNeedsPaintInvalidation(ScrollbarPart parts)
 {
-    if (m_theme.shouldRepaintAllPartsOnInvalidation()) {
+    if (m_theme.shouldRepaintAllPartsOnInvalidation())
+        parts = AllParts;
+    if (parts & ~ThumbPart)
         m_trackNeedsRepaint = true;
+    if (parts & ThumbPart)
         m_thumbNeedsRepaint = true;
-    }
     if (m_scrollableArea)
         m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }