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..1b735f104b2d23c200b07fccdfa3250e5689f907 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -839,6 +839,31 @@ 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) {
+        ScrollbarOverlayStyle oldOverlayStyle = scrollbarOverlayStyle();
+        ScrollbarOverlayStyle overlayStyle = ScrollbarOverlayStyleDefault;
+
+        // Reduce the background color from RGB to a lightness value
+        // and determine which scrollbar style to use based on a lightness
+        // heuristic.
+        double hue, saturation, lightness;
+        newBackground.getHSL(hue, saturation, lightness);
+        if (lightness <= .5)
+            overlayStyle = ScrollbarOverlayStyleLight;
+
+        if (oldOverlayStyle != overlayStyle)
+            setScrollbarOverlayStyle(overlayStyle);
+    }

[Xianzhu, 2016/02/24 23:48:26]

Can you extract common code in here and in FrameView into ScrollArea (e.g.
recalculateScrollbarOverlayStyle(Color backgroundColor))? Then you can just unit
test the above function in ScrollableAreaTest.

 }
 
 bool PaintLayerScrollableArea::updateAfterCompositingChange()
@@ -1481,9 +1506,14 @@ void PaintLayerScrollableArea::ScrollbarManager::setHasHorizontalScrollbar(bool
         // 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)
+        if (!m_hBar) {
             m_hBar = createScrollbar(HorizontalScrollbar);
-        m_hBarIsAttached = 1;
+            m_hBarIsAttached = 1;
+            m_scrollableArea->didAddScrollbar(*m_hBar, HorizontalScrollbar);
+        } else {
+            m_hBarIsAttached = 1;
+        }
+
     } else {
         m_hBarIsAttached = 0;
         if (!m_canDetachScrollbars)
@@ -1495,9 +1525,14 @@ void PaintLayerScrollableArea::ScrollbarManager::setHasVerticalScrollbar(bool ha
 {
     if (hasScrollbar) {
         DisableCompositingQueryAsserts disabler;
-        if (!m_vBar)
+        if (!m_vBar) {
             m_vBar = createScrollbar(VerticalScrollbar);
-        m_vBarIsAttached = 1;
+            m_vBarIsAttached = 1;
+            m_scrollableArea->didAddScrollbar(*m_vBar, VerticalScrollbar);
+        } else {
+            m_vBarIsAttached = 1;
+        }
+
     } else {
         m_vBarIsAttached = 0;
         if (!m_canDetachScrollbars)
@@ -1518,10 +1553,6 @@ PassRefPtrWillBeRawPtr<Scrollbar> PaintLayerScrollableArea::ScrollbarManager::cr
         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();