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..a18bf9080e377e08a1c3ec600d33c2c61b97b5f0 100644
--- a/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
+++ b/Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp
@@ -646,6 +646,19 @@ void DeprecatedPaintLayerScrollableArea::updateAfterLayout()
 {
     ASSERT(box().hasOverflowClip());
 
+    bool needsHBarConstruction = false;
+    bool needsVBarConstruction = false;
+    if (needsScrollbarReconstruction()) {
+        if (m_hBar) {
+            destroyScrollbar(HorizontalScrollbar);
+            needsHBarConstruction = true;

[skobes, 2015/08/14 19:02:56]

I don't understand the need to store this.  Why doesn't the existing
overflow:auto logic create the scrollbar?

+        }
+        if (m_vBar) {
+            destroyScrollbar(VerticalScrollbar);
+            needsVBarConstruction = true;
+        }
+    }
+
     DoubleSize originalScrollOffset = adjustedScrollOffset();
     computeScrollDimensions();
 
@@ -679,14 +692,15 @@ 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);
+    // 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 may also need to layout again.
+    bool horizontalScrollBarChanged = (box().hasAutoHorizontalScrollbar() && (hasHorizontalScrollbar() != hasHorizontalOverflow)) || needsHBarConstruction;

[skobes, 2015/08/14 19:23:37]

Oh I see, the problem is with overflow:scroll and not overflow:auto.

I think you can still avoid the needsHBarConstruction and needsVBarConstruction
vars.  Can we use

bool horizontalScrollBarChanged = (box().hasAutoHorizontalScrollbar() &&
(hasHorizontalScrollbar() != hasHorizontalOverflow))
    || (box().style()->overflowX() == OSCROLL && !horizontalScrollbar());
...?

[MuVen, 2015/08/17 09:21:03]

Done.

+    bool verticalScrollBarChanged = (box().hasAutoVerticalScrollbar() && (hasVerticalScrollbar() != hasVerticalOverflow)) || needsVBarConstruction;
 
-    if (!visualViewportSuppliesScrollbars() && (autoHorizontalScrollBarChanged || autoVerticalScrollBarChanged)) {
-        if (box().hasAutoHorizontalScrollbar())
+    if (!visualViewportSuppliesScrollbars() && (horizontalScrollBarChanged || verticalScrollBarChanged)) {
+        if (box().hasAutoHorizontalScrollbar() || needsHBarConstruction)
             setHasHorizontalScrollbar(hasHorizontalOverflow);
-        if (box().hasAutoVerticalScrollbar())
+        if (box().hasAutoVerticalScrollbar() || needsVBarConstruction)
             setHasVerticalScrollbar(hasVerticalOverflow);
 
         if (hasVerticalOverflow || hasHorizontalOverflow)
@@ -698,7 +712,7 @@ 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 || needsHBarConstruction || needsVBarConstruction) {
             if (!m_inOverflowRelayout) {
                 // Our proprietary overflow: overlay value doesn't trigger a layout.
                 m_inOverflowRelayout = true;
@@ -706,7 +720,7 @@ 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 +972,15 @@ 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;