Create custom scrollbars when html element has overflow:scroll

Source/core/paint/DeprecatedPaintLayerScrollableArea.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@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 628 matching lines @@
     if (newScrollOffset != adjustedScrollOffset()) {
         DoublePoint origin(scrollOrigin());
         ScrollableArea::setScrollPosition(-origin + newScrollOffset, ProgrammaticScroll, scrollBehavior);
     }
 }
 
 void DeprecatedPaintLayerScrollableArea::updateAfterLayout()
 {
     ASSERT(box().hasOverflowClip());
 
+    bool isScrollbarReconstructionRequired = needsScrollbarReconstruction();
+    if (m_hBar && !m_hBar->isCustomScrollbar() && isScrollbarReconstructionRequired) {
+        destroyScrollbar(HorizontalScrollbar);

[MuVen, 2015/08/13 19:36:32]

!m_hBar->isCustomScrollbar() This condition wont trigger reconstruction
every-time after layout. May be i see i need to pull all these conditions in the
needsScrollbarReconstruction logic. wdyt about the api calls in the if loop. is
that fine to call destroyScrollbar and call setHasHorizontalScrollbar(true) ? or

setHasHorizontalScrollbar(false);
setHasHorizontalScrollbar(true);

please share your inputs.

[skobes, 2015/08/13 19:53:43]

Yes, to make it easier to follow, needsScrollbarReconstruction should return
true if and only if one or both scrollbars need to be reconstructed.
I think if any scrollbar needs reconstruction it should be sufficient to destroy
them both, then let the regular overflow logic create them again below.

+        setHasHorizontalScrollbar(true);
+    }
+    if (m_vBar && !m_vBar->isCustomScrollbar() && isScrollbarReconstructionRequired) {
+        destroyScrollbar(VerticalScrollbar);
+        setHasVerticalScrollbar(true);
+    }
+
     DoubleSize originalScrollOffset = adjustedScrollOffset();
     computeScrollDimensions();
 
     // Layout may cause us to be at an invalid scroll position. In this case we need
     // to pull our scroll offsets back to the max (or push them up to the min).
     DoubleSize clampedScrollOffset = clampScrollOffset(adjustedScrollOffset());
     if (clampedScrollOffset != adjustedScrollOffset())
         scrollToOffset(clampedScrollOffset);
 
     if (originalScrollOffset != adjustedScrollOffset()) {
@@ skipping 292 matching lines @@
         }
         if (ShadowRoot* shadowRoot = node->containingShadowRoot()) {
             if (shadowRoot->type() == ShadowRootType::UserAgent)
                 return shadowRoot->host()->layoutObject();
         }
     }
 
     return &layoutObject;
 }
 
+bool DeprecatedPaintLayerScrollableArea::needsScrollbarReconstruction() const
+{
+    LayoutObject* actualLayoutObject = layoutObjectForScrollbar(box());
+    return actualLayoutObject->isBox() && actualLayoutObject->style()->hasPseudoStyle(SCROLLBAR);

[skobes, 2015/08/13 18:12:40]

This will reconstruct custom scrollbars after every layout.  We only want to
reconstruct if the scrollbar is currently native, and should be custom, or if it
is currently custom, and should be native.

+}
+
 PassRefPtrWillBeRawPtr<Scrollbar> DeprecatedPaintLayerScrollableArea::createScrollbar(ScrollbarOrientation orientation)
 {
     RefPtrWillBeRawPtr<Scrollbar> widget = nullptr;
     LayoutObject* actualLayoutObject = layoutObjectForScrollbar(box());
     bool hasCustomScrollbarStyle = actualLayoutObject->isBox() && actualLayoutObject->style()->hasPseudoStyle(SCROLLBAR);
     if (hasCustomScrollbarStyle) {
         widget = LayoutScrollbar::createCustomScrollbar(this, orientation, actualLayoutObject->node());
     } else {
         ScrollbarControlSize scrollbarSize = RegularScrollbar;
         if (actualLayoutObject->style()->hasAppearance())
@@ skipping 480 matching lines @@
         return false;
 
     LocalFrame* frame = box().frame();
     if (!frame || !frame->isMainFrame() || !frame->settings())
         return false;
 
     return frame->settings()->viewportMetaEnabled();
 }
 
 } // namespace blink