Blink Support for Overlay Scrollbar Animation

Source/platform/scroll/ScrollbarThemeOverlay.cpp

Index: Source/platform/scroll/ScrollbarThemeOverlay.cpp
diff --git a/Source/platform/scroll/ScrollbarThemeOverlay.cpp b/Source/platform/scroll/ScrollbarThemeOverlay.cpp
index a8a9556d992fea9853b8a3c861fbfe6e48233192..39ad2deedbb340ecb917d0233b9b5762e19bd6e3 100644
--- a/Source/platform/scroll/ScrollbarThemeOverlay.cpp
+++ b/Source/platform/scroll/ScrollbarThemeOverlay.cpp
@@ -36,6 +36,12 @@
 
 #include <algorithm>
 
+namespace {
+
+const int kDefaultOverlayScrollbarMinThumbThickness = 9;
+
+} // namespace
+
 namespace blink {
 
 ScrollbarThemeOverlay::ScrollbarThemeOverlay(int thumbThickness, int scrollbarMargin, HitTestBehavior allowHitTest, Color color)
@@ -136,19 +142,56 @@ void ScrollbarThemeOverlay::paintThumb(GraphicsContext* context, ScrollbarThemeC
         return;
     }
 
-    blink::WebThemeEngine::State state = blink::WebThemeEngine::StateNormal;
-    if (scrollbar->pressedPart() == ThumbPart)
-        state = blink::WebThemeEngine::StatePressed;
-    else if (scrollbar->hoveredPart() == ThumbPart)
-        state = blink::WebThemeEngine::StateHover;
-
     blink::WebCanvas* canvas = context->canvas();
 
     blink::WebThemeEngine::Part part = blink::WebThemeEngine::PartScrollbarHorizontalThumb;
     if (scrollbar->orientation() == VerticalScrollbar)
         part = blink::WebThemeEngine::PartScrollbarVerticalThumb;
 
-    blink::Platform::current()->themeEngine()->paint(canvas, part, state, blink::WebRect(rect), 0);
+    if (scrollbar->isDuringStateTransitionAnimation()) {
+        // Paint in between of two states. First get information from ScrollAnimator
+        double progress = scrollbar->stateTransitionProgress();
+        blink::WebThemeEngine::State startState = scrollbar->stateTransitionStartState();
+        blink::WebThemeEngine::State endState = scrollbar->stateTransitionEndState();
+        double thumbThicknessChanged = progress * (m_thumbThickness - kDefaultOverlayScrollbarMinThumbThickness);
+        double disThumbThickness = m_thumbThickness - thumbThicknessChanged;
+        int displayedThumbThickness = static_cast<int>(disThumbThickness);
+        if (startState == blink::WebThemeEngine::StateNormal)
+            displayedThumbThickness = kDefaultOverlayScrollbarMinThumbThickness + thumbThicknessChanged;
+
+        if (scrollbar->orientation() == HorizontalScrollbar) {
+            thumbRect.setY(thumbRect.y() + (m_thumbThickness - displayedThumbThickness));
+            thumbRect.setHeight(displayedThumbThickness);
+        } else {
+            thumbRect.setWidth(displayedThumbThickness);
+            if (!scrollbar->isLeftSideVerticalScrollbar())
+                thumbRect.setX(thumbRect.x() + (m_thumbThickness - displayedThumbThickness));
+        }
+
+        //        int testThumbThickness = static_cast<int>(displayedThumbThickness);
+        // thumbRect.setWidth(1);
+
+        blink::Platform::current()->themeEngine()->paintStateTransition(canvas, part, startState, endState, progress, blink::WebRect(thumbRect));
+        return;
+    }
+
+    blink::WebThemeEngine::State state = blink::WebThemeEngine::StateNormal;
+    if (scrollbar->pressedPart() == ThumbPart) {
+        state = blink::WebThemeEngine::StatePressed;
+    } else if (scrollbar->hoveredPart() == ThumbPart) {
+        state = blink::WebThemeEngine::StateHover;
+    } else {
+        if (scrollbar->orientation() == HorizontalScrollbar) {
+            thumbRect.setY(thumbRect.y() + (m_thumbThickness - kDefaultOverlayScrollbarMinThumbThickness));
+            thumbRect.setHeight(kDefaultOverlayScrollbarMinThumbThickness);
+        } else {
+            thumbRect.setWidth(kDefaultOverlayScrollbarMinThumbThickness);
+            if (!scrollbar->isLeftSideVerticalScrollbar())
+                thumbRect.setX(thumbRect.x() + (m_thumbThickness - kDefaultOverlayScrollbarMinThumbThickness));
+        }
+    }
+
+    blink::Platform::current()->themeEngine()->paint(canvas, part, state, blink::WebRect(thumbRect), 0);
 }
 
 ScrollbarPart ScrollbarThemeOverlay::hitTest(ScrollbarThemeClient* scrollbar, const IntPoint& position)