Fixed Inconsistent behaviour of custom-scrollbar

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 3432 matching lines @@
     // If we ever turn one scrollbar off, always turn the other one off too.
     // Never ever try to both gain/lose a scrollbar in the same pass.
     if (!newHasHorizontalScrollbar && hasHorizontalScrollbar && vScroll != ScrollbarAlwaysOn)
         newHasVerticalScrollbar = false;
     if (!newHasVerticalScrollbar && hasVerticalScrollbar && hScroll != ScrollbarAlwaysOn)
         newHasHorizontalScrollbar = false;
 }
 
 void FrameView::updateScrollbarGeometry()
 {
+    bool scrollbarExistenceChanged = false;

[pdr., 2014/10/14 22:02:21]

Is the scrollbar existence changing, or just the size?

     if (m_horizontalScrollbar) {
         int clientWidth = visibleWidth();
         IntRect oldRect(m_horizontalScrollbar->frameRect());
         IntRect hBarRect((shouldPlaceVerticalScrollbarOnLeft() && m_verticalScrollbar) ? m_verticalScrollbar->width() : 0,
             height() - m_horizontalScrollbar->height(),
             width() - (m_verticalScrollbar ? m_verticalScrollbar->width() : 0),
             m_horizontalScrollbar->height());
         m_horizontalScrollbar->setFrameRect(adjustScrollbarRectForResizer(hBarRect, m_horizontalScrollbar.get()));
         if (!m_scrollbarsSuppressed && oldRect != m_horizontalScrollbar->frameRect())
             m_horizontalScrollbar->invalidate();
 
+        oldRect = m_horizontalScrollbar->frameRect();

[pdr., 2014/10/14 22:02:21]

Why do you need to store the oldRect here instead of using the value from above?

         if (m_scrollbarsSuppressed)
             m_horizontalScrollbar->setSuppressInvalidation(true);
         m_horizontalScrollbar->setEnabled(contentsWidth() > clientWidth);
         m_horizontalScrollbar->setProportion(clientWidth, contentsWidth());
         m_horizontalScrollbar->offsetDidChange();
         if (m_scrollbarsSuppressed)
             m_horizontalScrollbar->setSuppressInvalidation(false);
+        if (oldRect != m_horizontalScrollbar->frameRect())
+            scrollbarExistenceChanged = true;
     }
 
     if (m_verticalScrollbar) {
         int clientHeight = visibleHeight();
         IntRect oldRect(m_verticalScrollbar->frameRect());
         IntRect vBarRect(shouldPlaceVerticalScrollbarOnLeft() ? 0 : (width() - m_verticalScrollbar->width()),
             0,
             m_verticalScrollbar->width(),
             height() - (m_horizontalScrollbar ? m_horizontalScrollbar->height() : 0));
         m_verticalScrollbar->setFrameRect(adjustScrollbarRectForResizer(vBarRect, m_verticalScrollbar.get()));
         if (!m_scrollbarsSuppressed && oldRect != m_verticalScrollbar->frameRect())
             m_verticalScrollbar->invalidate();
 
+        oldRect = m_verticalScrollbar->frameRect();
         if (m_scrollbarsSuppressed)
             m_verticalScrollbar->setSuppressInvalidation(true);
         m_verticalScrollbar->setEnabled(contentsHeight() > clientHeight);
         m_verticalScrollbar->setProportion(clientHeight, contentsHeight());
         m_verticalScrollbar->offsetDidChange();
         if (m_scrollbarsSuppressed)
             m_verticalScrollbar->setSuppressInvalidation(false);
+        if (oldRect != m_verticalScrollbar->frameRect())

[pdr., 2014/10/14 22:02:21]

I don't understand the values we're seeing here. On my machine, I see the
following:

zoom at 100%: height is 275px
zoom at 125%: height is 275px
zoom at 150%: height is 275px
zoom at 175%: height is 275px
zoom at 200%: height is 275px
zoom at 250%: height is 273px
zoom at 300%: height is 273px

Something doesn't seem right here.

+            scrollbarExistenceChanged = true;
     }
+
+    if (scrollbarExistenceChanged)
+        scrollbarExistenceDidChange();
 }
 
 IntRect FrameView::adjustScrollbarRectForResizer(const IntRect& rect, Scrollbar* scrollbar)
 {
     // Get our window resizer rect and see if we overlap. Adjust to avoid the overlap
     // if necessary.
     IntRect adjustedRect(rect);
     bool overlapsResizer = false;
     if (!rect.isEmpty() && !windowResizerRect().isEmpty()) {
         IntRect resizerRect = convertFromContainingWindow(windowResizerRect());
@@ skipping 721 matching lines @@
         return;
 
     ScrollableArea::setScrollOrigin(origin);
 
     // Update if the scroll origin changes, since our position will be different if the content size did not change.
     if (updatePositionAtAll && updatePositionSynchronously)
         updateScrollbars(scrollOffsetDouble());
 }
 
 } // namespace blink