Overlay scrollbars flush with window edge

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 a3dba61d2e9123793929f28f84c13ddbbc9a166e..df8bee90fd3e601336d520763d395331406ec1fe 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -63,7 +63,8 @@ Scrollbar::Scrollbar(ScrollableArea* scrollableArea,
                     &Scrollbar::autoscrollTimerFired),
       m_elasticOverscroll(0),
       m_trackNeedsRepaint(true),
-      m_thumbNeedsRepaint(true) {
+      m_thumbNeedsRepaint(true),
+      m_hitBeginOrEnd(true) {
   m_theme.registerScrollbar(*this);
 
   // FIXME: This is ugly and would not be necessary if we fix cross-platform
@@ -130,8 +131,14 @@ void Scrollbar::offsetDidChange() {
   int oldThumbPosition = theme().thumbPosition(*this);
   m_currentPos = position;
 
-  ScrollbarPart invalidParts =
-      theme().invalidateOnThumbPositionChange(*this, oldPosition, position);
+  bool oldHitBeginOrEnd = m_hitBeginOrEnd;
+  int thumbPosition = theme().thumbPosition(*this);
+  m_hitBeginOrEnd =
+      thumbPosition == 0 ||
+      thumbPosition + theme().thumbLength(*this) == theme().trackLength(*this);
+
+  ScrollbarPart invalidParts = theme().invalidateOnThumbPositionChange(
+      *this, oldPosition, position, oldHitBeginOrEnd != m_hitBeginOrEnd);
   setNeedsPaintInvalidation(invalidParts);
 
   if (m_pressedPart == ThumbPart)
@@ -647,4 +654,8 @@ void Scrollbar::setNeedsPaintInvalidation(ScrollbarPart invalidParts) {
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
+bool Scrollbar::hasScrollCorner() const {
+  return m_scrollableArea->hasLayerForScrollCorner();
+}
+
 }  // namespace blink