Overlay scrollbars flush with window edge

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
index 30ba2b9554699d95115ee07d6d3ed8f1506b2dd7..6898373b6088f859477b4a556fc7c92eb362d3b3 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.cpp
@@ -65,7 +65,10 @@ bool ScrollbarThemeOverlay::shouldRepaintAllPartsOnInvalidation() const {
 ScrollbarPart ScrollbarThemeOverlay::invalidateOnThumbPositionChange(
     const ScrollbarThemeClient&,
     float oldPosition,
-    float newPosition) const {
+    float newPosition,
+    bool hitBeginOrEndChanged) const {
+  if (hitBeginOrEndChanged)
+    return ThumbPart;
   return NoPart;
 }
 
@@ -204,6 +207,17 @@ void ScrollbarThemeOverlay::paintThumb(GraphicsContext& context,
   params.scrollbarThumb.scrollbarTheme =
       static_cast<WebScrollbarOverlayColorTheme>(
           scrollbar.getScrollbarOverlayColorTheme());
+  params.scrollbarThumb.isVerticalScrollbar =
+      scrollbar.orientation() == VerticalScrollbar;
+  params.scrollbarThumb.isLeftVerticalScrollbar =
+      scrollbar.isLeftSideVerticalScrollbar();
+
+  int currentThumbPosition = ScrollbarTheme::thumbPosition(scrollbar);
+
+  params.scrollbarThumb.hitStart = currentThumbPosition == 0;
+  params.scrollbarThumb.hitEnd =
+      !scrollbar.hasScrollCorner() &&
+      currentThumbPosition + thumbLength(scrollbar) == trackLength(scrollbar);
 
   Platform::current()->themeEngine()->paint(canvas, part, state, WebRect(rect),
                                             &params);