Fix overlay scroll bar color on elements with dark background

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 821 matching lines @@
 
     // FIXME: Need to detect a swap from custom to native scrollbars (and vice versa).
     if (horizontalScrollbar())
         horizontalScrollbar()->styleChanged();
     if (verticalScrollbar())
         verticalScrollbar()->styleChanged();
 
     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()
 {
     layer()->updateScrollingStateAfterCompositingChange();
     const bool layersChanged = m_topmostScrollChild != m_nextTopmostScrollChild;
     m_topmostScrollChild = m_nextTopmostScrollChild;
     m_nextTopmostScrollChild = nullptr;
     return layersChanged;
 }
@@ skipping 622 matching lines @@
             destroyScrollbar(VerticalScrollbar);
     }
 }
 
 void PaintLayerScrollableArea::ScrollbarManager::setHasHorizontalScrollbar(bool hasScrollbar)
 {
     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)
+        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)
             destroyScrollbar(HorizontalScrollbar);
     }
 }
 
 void PaintLayerScrollableArea::ScrollbarManager::setHasVerticalScrollbar(bool hasScrollbar)
 {
     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)
             destroyScrollbar(VerticalScrollbar);
     }
 }
 
 PassRefPtrWillBeRawPtr<Scrollbar> PaintLayerScrollableArea::ScrollbarManager::createScrollbar(ScrollbarOrientation orientation)
 {
     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) {
         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();
 }
 
 void PaintLayerScrollableArea::ScrollbarManager::destroyScrollbar(ScrollbarOrientation orientation)
 {
     RefPtrWillBeMember<Scrollbar>& scrollbar = orientation == HorizontalScrollbar ? m_hBar : m_vBar;
     ASSERT(orientation == HorizontalScrollbar ? !m_hBarIsAttached: !m_vBarIsAttached);
     if (!scrollbar)
@@ skipping 14 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink