Reland: Make ScrollbarThemeAura selectively invalidate scrollbar parts.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
index f434c09ca5353e8f67d9583e751d403cc1de6034..5e238eff0c144713da5cc1ee90ca55846d2058a5 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarThemeAura.cpp
@@ -43,11 +43,89 @@
 
 namespace blink {
 
+namespace {
+
 static bool useMockTheme()
 {
     return LayoutTestSupport::isRunningLayoutTest();
 }
 
+// Contains a flag indicating whether WebThemeEngine should paint a UI widget
+// for a scrollbar part, and if so, what part and state apply.
+//
+// If the PartPaintingParams are not affected by a change in the scrollbar
+// state, then the corresponding scrollbar part does not need to be repainted.
+struct PartPaintingParams {
+    PartPaintingParams()
+        : shouldPaint(false)
+        , part(WebThemeEngine::PartScrollbarDownArrow)
+        , state(WebThemeEngine::StateNormal) {}
+    PartPaintingParams(WebThemeEngine::Part part, WebThemeEngine::State state)
+        : shouldPaint(true)
+        , part(part)
+        , state(state) {}
+
+    bool shouldPaint;
+    WebThemeEngine::Part part;
+    WebThemeEngine::State state;
+};
+
+bool operator==(const PartPaintingParams& a, const PartPaintingParams& b)
+{
+    return (!a.shouldPaint && !b.shouldPaint) || std::tie(a.shouldPaint, a.part, a.state) == std::tie(b.shouldPaint, b.part, b.state);
+}
+
+bool operator!=(const PartPaintingParams& a, const PartPaintingParams& b)
+{
+    return !(a == b);
+}
+
+PartPaintingParams buttonPartPaintingParams(const ScrollbarThemeClient& scrollbar, float position, ScrollbarPart part)
+{
+    WebThemeEngine::Part paintPart;
+    WebThemeEngine::State state = WebThemeEngine::StateNormal;
+    bool checkMin = false;
+    bool checkMax = false;
+
+    if (scrollbar.orientation() == HorizontalScrollbar) {
+        if (part == BackButtonStartPart) {
+            paintPart = WebThemeEngine::PartScrollbarLeftArrow;
+            checkMin = true;
+        } else if (useMockTheme() && part != ForwardButtonEndPart) {
+            return PartPaintingParams();
+        } else {
+            paintPart = WebThemeEngine::PartScrollbarRightArrow;
+            checkMax = true;
+        }
+    } else {
+        if (part == BackButtonStartPart) {
+            paintPart = WebThemeEngine::PartScrollbarUpArrow;
+            checkMin = true;
+        } else if (useMockTheme() && part != ForwardButtonEndPart) {
+            return PartPaintingParams();
+        } else {
+            paintPart = WebThemeEngine::PartScrollbarDownArrow;
+            checkMax = true;
+        }
+    }
+
+    if (useMockTheme() && !scrollbar.enabled()) {
+        state = WebThemeEngine::StateDisabled;
+    } else if (!useMockTheme() && ((checkMin && (position <= 0))
+        || (checkMax && position >= scrollbar.maximum()))) {
+        state = WebThemeEngine::StateDisabled;
+    } else {
+        if (part == scrollbar.pressedPart())
+            state = WebThemeEngine::StatePressed;
+        else if (part == scrollbar.hoveredPart())
+            state = WebThemeEngine::StateHover;
+    }
+
+    return PartPaintingParams(paintPart, state);
+}
+
+} // namespace
+
 ScrollbarTheme& ScrollbarTheme::nativeTheme()
 {
     if (RuntimeEnabledFeatures::overlayScrollbarsEnabled()) {
@@ -69,6 +147,24 @@ int ScrollbarThemeAura::scrollbarThickness(ScrollbarControlSize controlSize)
     return scrollbarSize.width();
 }
 
+bool ScrollbarThemeAura::shouldRepaintAllPartsOnInvalidation() const
+{
+    // This theme can separately handle thumb invalidation.
+    return false;
+}
+
+ScrollbarPart ScrollbarThemeAura::invalidateOnThumbPositionChange(const ScrollbarThemeClient& scrollbar, float oldPosition, float newPosition) const
+{
+    ScrollbarPart invalidParts = NoPart;
+    ASSERT(buttonsPlacement() == WebScrollbarButtonsPlacementSingle);
+    static const ScrollbarPart kButtonParts[] = {BackButtonStartPart, ForwardButtonEndPart};
+    for (ScrollbarPart part : kButtonParts) {
+        if (buttonPartPaintingParams(scrollbar, oldPosition, part) != buttonPartPaintingParams(scrollbar, newPosition, part))
+            invalidParts = static_cast<ScrollbarPart>(invalidParts | part);
+    }
+    return invalidParts;
+}
+
 void ScrollbarThemeAura::paintTrackPiece(GraphicsContext& gc, const ScrollbarThemeClient& scrollbar, const IntRect& rect, ScrollbarPart partType)
 {
     DisplayItem::Type displayItemType = trackPiecePartToDisplayItemType(partType);
@@ -94,51 +190,14 @@ void ScrollbarThemeAura::paintTrackPiece(GraphicsContext& gc, const ScrollbarThe
 
 void ScrollbarThemeAura::paintButton(GraphicsContext& gc, const ScrollbarThemeClient& scrollbar, const IntRect& rect, ScrollbarPart part)
 {
-    WebThemeEngine::Part paintPart;
-    WebThemeEngine::State state = WebThemeEngine::StateNormal;
-    bool checkMin = false;
-    bool checkMax = false;
-
-    if (scrollbar.orientation() == HorizontalScrollbar) {
-        if (part == BackButtonStartPart) {
-            paintPart = WebThemeEngine::PartScrollbarLeftArrow;
-            checkMin = true;
-        } else if (useMockTheme() && part != ForwardButtonEndPart) {
-            return;
-        } else {
-            paintPart = WebThemeEngine::PartScrollbarRightArrow;
-            checkMax = true;
-        }
-    } else {
-        if (part == BackButtonStartPart) {
-            paintPart = WebThemeEngine::PartScrollbarUpArrow;
-            checkMin = true;
-        } else if (useMockTheme() && part != ForwardButtonEndPart) {
-            return;
-        } else {
-            paintPart = WebThemeEngine::PartScrollbarDownArrow;
-            checkMax = true;
-        }
-    }
-
     DisplayItem::Type displayItemType = buttonPartToDisplayItemType(part);
     if (DrawingRecorder::useCachedDrawingIfPossible(gc, scrollbar, displayItemType))
         return;
-
+    PartPaintingParams params = buttonPartPaintingParams(scrollbar, scrollbar.currentPos(), part);
+    if (!params.shouldPaint)
+        return;
     DrawingRecorder recorder(gc, scrollbar, displayItemType, rect);
-
-    if (useMockTheme() && !scrollbar.enabled()) {
-        state = WebThemeEngine::StateDisabled;
-    } else if (!useMockTheme() && ((checkMin && (scrollbar.currentPos() <= 0))
-        || (checkMax && scrollbar.currentPos() >= scrollbar.maximum()))) {
-        state = WebThemeEngine::StateDisabled;
-    } else {
-        if (part == scrollbar.pressedPart())
-            state = WebThemeEngine::StatePressed;
-        else if (part == scrollbar.hoveredPart())
-            state = WebThemeEngine::StateHover;
-    }
-    Platform::current()->themeEngine()->paint(gc.canvas(), paintPart, state, WebRect(rect), 0);
+    Platform::current()->themeEngine()->paint(gc.canvas(), params.part, params.state, WebRect(rect), 0);
 }
 
 void ScrollbarThemeAura::paintThumb(GraphicsContext& gc, const ScrollbarThemeClient& scrollbar, const IntRect& rect)