Fix overlay scroll bar color on elements with dark background

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index ba2878b3f7468b7f66b5582495755e38850c1913..429ac98381cff679198e280227d64a6c4a0fdc7b 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -839,6 +839,19 @@ void PaintLayerScrollableArea::updateAfterStyleChange(const ComputedStyle* oldSt
     updateScrollCornerStyle();
     updateResizerAreaSet();
     updateResizerStyle();
+
+    // Whenever background changes on the scrollable element, the scroll bar
+    // overlay style might need to be changed to have contrast against the
+    // background.
+    Color oldBackground;
+    if (oldStyle) {
+        oldBackground = oldStyle->visitedDependentColor(CSSPropertyBackgroundColor);
+    }
+    Color newBackground = box().style()->visitedDependentColor(CSSPropertyBackgroundColor);
+
+    if (newBackground != oldBackground) {
+        recalculateScrollbarOverlayStyle(newBackground);
+    }
 }
 
 bool PaintLayerScrollableArea::updateAfterCompositingChange()
@@ -961,10 +974,16 @@ static inline const LayoutObject& layoutObjectForScrollbar(const LayoutObject& l
     return layoutObject;
 }
 
+bool PaintLayerScrollableArea::useCustomScrollbarStyle() const
+{
+    const LayoutObject& actualLayoutObject = layoutObjectForScrollbar(box());
+    return actualLayoutObject.isBox() && actualLayoutObject.styleRef().hasPseudoStyle(SCROLLBAR);
+}
+
 bool PaintLayerScrollableArea::needsScrollbarReconstruction() const
 {
     const LayoutObject& actualLayoutObject = layoutObjectForScrollbar(box());
-    bool shouldUseCustom = actualLayoutObject.isBox() && actualLayoutObject.styleRef().hasPseudoStyle(SCROLLBAR);
+    bool shouldUseCustom = useCustomScrollbarStyle();
     bool hasAnyScrollbar = hasScrollbar();
     bool hasCustom = (hasHorizontalScrollbar() && horizontalScrollbar()->isCustomScrollbar()) || (hasVerticalScrollbar() && verticalScrollbar()->isCustomScrollbar());
     bool didCustomScrollbarOwnerChanged = false;
@@ -989,7 +1008,7 @@ void PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar)
 
     setScrollbarNeedsPaintInvalidation(HorizontalScrollbar);
 
-    m_scrollbarManager.setHasHorizontalScrollbar(hasScrollbar);
+    m_scrollbarManager.setHasHorizontalScrollbar(hasScrollbar, useCustomScrollbarStyle());
 
     // Destroying or creating one bar can cause our scrollbar corner to come and go. We need to update the opposite scrollbar's style.
     if (hasHorizontalScrollbar())
@@ -1011,7 +1030,7 @@ void PaintLayerScrollableArea::setHasVerticalScrollbar(bool hasScrollbar)
 
     setScrollbarNeedsPaintInvalidation(VerticalScrollbar);
 
-    m_scrollbarManager.setHasVerticalScrollbar(hasScrollbar);
+    m_scrollbarManager.setHasVerticalScrollbar(hasScrollbar, useCustomScrollbarStyle());
 
     // Destroying or creating one bar can cause our scrollbar corner to come and go. We need to update the opposite scrollbar's style.
     if (hasHorizontalScrollbar())
@@ -1475,15 +1494,21 @@ void PaintLayerScrollableArea::ScrollbarManager::setCanDetachScrollbars(bool det
     }
 }
 
-void PaintLayerScrollableArea::ScrollbarManager::setHasHorizontalScrollbar(bool hasScrollbar)
+void PaintLayerScrollableArea::ScrollbarManager::setHasHorizontalScrollbar(bool hasScrollbar, bool useCustomScrollbarStyle)
 {
     if (hasScrollbar) {
         // This doesn't hit in any tests, but since the equivalent code in setHasVerticalScrollbar
         // does, presumably this code does as well.
         DisableCompositingQueryAsserts disabler;
-        if (!m_hBar)
-            m_hBar = createScrollbar(HorizontalScrollbar);
-        m_hBarIsAttached = 1;
+        if (!m_hBar) {
+            m_hBar = createScrollbar(HorizontalScrollbar, useCustomScrollbarStyle);
+            m_hBarIsAttached = 1;
+            if (!useCustomScrollbarStyle)
+                m_scrollableArea->didAddScrollbar(*m_hBar, HorizontalScrollbar);

[Xianzhu, 2016/03/02 00:03:50]

I suggest to check the condition in ScrollableArea::didAddScrollbar() because
the function doesn't explicitly disallow custom scrollbars based on its name.

In ScrollbarArea::didAddScrollbar() you can add:

  if (scrollbar.isCustomScrollbar())
     return;

+        } else {
+            m_hBarIsAttached = 1;
+        }
+
     } else {
         m_hBarIsAttached = 0;
         if (!m_canDetachScrollbars)
@@ -1491,13 +1516,19 @@ void PaintLayerScrollableArea::ScrollbarManager::setHasHorizontalScrollbar(bool
     }
 }
 
-void PaintLayerScrollableArea::ScrollbarManager::setHasVerticalScrollbar(bool hasScrollbar)
+void PaintLayerScrollableArea::ScrollbarManager::setHasVerticalScrollbar(bool hasScrollbar, bool useCustomScrollbarStyle)
 {
     if (hasScrollbar) {
         DisableCompositingQueryAsserts disabler;
-        if (!m_vBar)
-            m_vBar = createScrollbar(VerticalScrollbar);
-        m_vBarIsAttached = 1;
+        if (!m_vBar) {
+            m_vBar = createScrollbar(VerticalScrollbar, useCustomScrollbarStyle);
+            m_vBarIsAttached = 1;
+            if (!useCustomScrollbarStyle)
+                m_scrollableArea->didAddScrollbar(*m_vBar, VerticalScrollbar);
+        } else {
+            m_vBarIsAttached = 1;
+        }
+
     } else {
         m_vBarIsAttached = 0;
         if (!m_canDetachScrollbars)
@@ -1505,23 +1536,18 @@ void PaintLayerScrollableArea::ScrollbarManager::setHasVerticalScrollbar(bool ha
     }
 }
 
-PassRefPtrWillBeRawPtr<Scrollbar> PaintLayerScrollableArea::ScrollbarManager::createScrollbar(ScrollbarOrientation orientation)
+PassRefPtrWillBeRawPtr<Scrollbar> PaintLayerScrollableArea::ScrollbarManager::createScrollbar(ScrollbarOrientation orientation, bool useCustomScrollbarStyle)

[Xianzhu, 2016/03/02 00:03:50]

I suggest to avoid the new parameter and call useCustomScrollbarStyle() here.
This avoids a bool parameter which is not suggested in our coding style.

 {
     ASSERT(orientation == HorizontalScrollbar ? !m_hBarIsAttached : !m_vBarIsAttached);
     RefPtrWillBeRawPtr<Scrollbar> scrollbar = nullptr;
     const LayoutObject& actualLayoutObject = layoutObjectForScrollbar(m_scrollableArea->box());
-    bool hasCustomScrollbarStyle = actualLayoutObject.isBox() && actualLayoutObject.styleRef().hasPseudoStyle(SCROLLBAR);
-    if (hasCustomScrollbarStyle) {
+    if (useCustomScrollbarStyle) {
         scrollbar = LayoutScrollbar::createCustomScrollbar(m_scrollableArea.get(), orientation, actualLayoutObject.node());
     } else {
         ScrollbarControlSize scrollbarSize = RegularScrollbar;
         if (actualLayoutObject.styleRef().hasAppearance())
             scrollbarSize = LayoutTheme::theme().scrollbarControlSizeForPart(actualLayoutObject.styleRef().appearance());
         scrollbar = Scrollbar::create(m_scrollableArea.get(), orientation, scrollbarSize, &m_scrollableArea->box().frame()->page()->chromeClient());
-        if (orientation == HorizontalScrollbar)
-            m_scrollableArea->didAddScrollbar(*scrollbar, HorizontalScrollbar);
-        else
-            m_scrollableArea->didAddScrollbar(*scrollbar, VerticalScrollbar);
     }
     m_scrollableArea->box().document().view()->addChild(scrollbar.get());
     return scrollbar.release();