Create custom scrollbars when html element has overflow:scroll

Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp

Index: Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
diff --git a/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp b/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
index 75b2999646e092109c931114f7a27add34ba4c50..f7f23f79bc10af40d01c98fd5125cc20acd816ec 100644
--- a/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
+++ b/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
@@ -646,6 +646,13 @@ void DeprecatedPaintLayerScrollableArea::updateAfterLayout()
 {
     ASSERT(box().hasOverflowClip());
 
+    if (needsScrollbarReconstruction()) {
+        if (m_hBar)
+            destroyScrollbar(HorizontalScrollbar);
+        if (m_vBar)
+            destroyScrollbar(VerticalScrollbar);
+    }
+
     DoubleSize originalScrollOffset = adjustedScrollOffset();
     computeScrollDimensions();
 
@@ -679,14 +686,14 @@ void DeprecatedPaintLayerScrollableArea::updateAfterLayout()
         if (!scrollSize(VerticalScrollbar))
             setHasVerticalScrollbar(false);
     }
-    // overflow:auto may need to lay out again if scrollbars got added/removed.
-    bool autoHorizontalScrollBarChanged = box().hasAutoHorizontalScrollbar() && (hasHorizontalScrollbar() != hasHorizontalOverflow);
-    bool autoVerticalScrollBarChanged = box().hasAutoVerticalScrollbar() && (hasVerticalScrollbar() != hasVerticalOverflow);
-
-    if (!visualViewportSuppliesScrollbars() && (autoHorizontalScrollBarChanged || autoVerticalScrollBarChanged)) {
-        if (box().hasAutoHorizontalScrollbar())
+    // overflow:auto may need to lay out again if scrollbars got added/removed and even on reconstruction of scrollbars
+    // from native to custom or vice versa also needs to layout again.
+    bool horizontalScrollBarChanged = (box().hasAutoHorizontalScrollbar() && (hasHorizontalScrollbar() != hasHorizontalOverflow)) || (box().style()->overflowX() == OSCROLL && !horizontalScrollbar());
+    bool verticalScrollBarChanged = (box().hasAutoVerticalScrollbar() && (hasVerticalScrollbar() != hasVerticalOverflow)) || (box().style()->overflowY() == OSCROLL && !verticalScrollbar());
+    if (!visualViewportSuppliesScrollbars() && (horizontalScrollBarChanged || verticalScrollBarChanged)) {
+        if (box().hasAutoHorizontalScrollbar() || (box().style()->overflowX() == OSCROLL && !horizontalScrollbar()))
             setHasHorizontalScrollbar(hasHorizontalOverflow);

[skobes, 2015/08/17 22:59:01]

This seems wrong for OSCROLL, which should show the scrollbar regardless of
whether there is overflow.

Perhaps you want:

if (box().style()->overflowX() == OSCROLL)
    setHasHorizontalScrollbar(true);

-        if (box().hasAutoVerticalScrollbar())
+        if (box().hasAutoVerticalScrollbar() || (box().style()->overflowY() == OSCROLL && !verticalScrollbar()))
             setHasVerticalScrollbar(hasVerticalOverflow);
 
         if (hasVerticalOverflow || hasHorizontalOverflow)
@@ -698,7 +705,7 @@ void DeprecatedPaintLayerScrollableArea::updateAfterLayout()
         if (box().document().hasAnnotatedRegions())
             box().document().setAnnotatedRegionsDirty(true);
 
-        if (box().style()->overflowX() == OAUTO || box().style()->overflowY() == OAUTO) {
+        if (box().style()->overflowX() == OAUTO || box().style()->overflowY() == OAUTO || box().style()->overflowX() == OSCROLL || box().style()->overflowY() == OSCROLL) {

[skobes, 2015/08/17 22:59:01]

Is this if statement even needed?  We are already guaranteed that
(horizontalScrollBarChanged || verticalScrollBarChanged) by the time we get
here.

[MuVen, 2015/08/18 14:38:53]

We may need this statement. Technically if we see, not required. But testcases
like fast/repaint/layout-state-only-positioned.html needs this statement.

             if (!m_inOverflowRelayout) {
                 // Our proprietary overflow: overlay value doesn't trigger a layout.
                 m_inOverflowRelayout = true;
@@ -706,7 +713,7 @@ void DeprecatedPaintLayerScrollableArea::updateAfterLayout()
                 layoutScope.setNeedsLayout(&box(), LayoutInvalidationReason::ScrollbarChanged);
                 if (box().isLayoutBlock()) {
                     LayoutBlock& block = toLayoutBlock(box());
-                    block.scrollbarsChanged(autoHorizontalScrollBarChanged, autoVerticalScrollBarChanged);
+                    block.scrollbarsChanged(horizontalScrollBarChanged, verticalScrollBarChanged);
                     block.layoutBlock(true);
                 } else {
                     box().layout();
@@ -958,6 +965,15 @@ static inline LayoutObject* layoutObjectForScrollbar(LayoutObject& layoutObject)
     return &layoutObject;
 }
 
+bool DeprecatedPaintLayerScrollableArea::needsScrollbarReconstruction() const
+{
+    LayoutObject* actualLayoutObject = layoutObjectForScrollbar(box());
+    bool shouldUseCustom = actualLayoutObject->isBox() && actualLayoutObject->style()->hasPseudoStyle(SCROLLBAR);
+    bool hasAnyScrollbar = hasScrollbar();
+    bool hasCustom = (m_hBar && m_hBar->isCustomScrollbar()) || (m_vBar && m_vBar->isCustomScrollbar());
+    return hasAnyScrollbar && (shouldUseCustom != hasCustom);
+}
+
 PassRefPtrWillBeRawPtr<Scrollbar> DeprecatedPaintLayerScrollableArea::createScrollbar(ScrollbarOrientation orientation)
 {
     RefPtrWillBeRawPtr<Scrollbar> widget = nullptr;