Fixed Inconsistent behaviour of custom-scrollbar

Source/core/frame/FrameView.cpp

Index: Source/core/frame/FrameView.cpp
diff --git a/Source/core/frame/FrameView.cpp b/Source/core/frame/FrameView.cpp
index d5bf83aa8aecf7602cb2c174a742c86203a68ac7..cf1a70c62357222d3c6572d2cbab835764ae4f5b 100644
--- a/Source/core/frame/FrameView.cpp
+++ b/Source/core/frame/FrameView.cpp
@@ -3299,6 +3299,8 @@ void FrameView::updateScrollbarGeometry()
         if (!m_scrollbarsSuppressed && oldRect != m_horizontalScrollbar->frameRect())
             m_horizontalScrollbar->invalidate();
 
+        oldRect = m_horizontalScrollbar->frameRect();
+
         if (m_scrollbarsSuppressed)
             m_horizontalScrollbar->setSuppressInvalidation(true);
         m_horizontalScrollbar->setEnabled(contentsWidth() > clientWidth);
@@ -3306,6 +3308,17 @@ void FrameView::updateScrollbarGeometry()
         m_horizontalScrollbar->offsetDidChange();
         if (m_scrollbarsSuppressed)
             m_horizontalScrollbar->setSuppressInvalidation(false);
+
+        if (!m_scrollbarsSuppressed && oldRect != m_horizontalScrollbar->frameRect() && m_horizontalScrollbar->isCustomScrollbar()) {
+            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 (oldRect.height() != m_horizontalScrollbar->frameRect().height())
+                scrollbarExistenceDidChange();

[esprehn, 2014/10/29 06:43:50]

What behavior are you trying to trigger here? The existence didn't change when
the height changes, this doesn't seem like the right call.

[MuVen, 2014/10/29 09:39:59]

Upon zooming in, customScrollbar style scales by scaleFactor. When
SetEnabled is called, updateScrollbarParts will be called with in the SetEnabled
function, which triggers RenderScrollbar::updateScrollbarPart. In this API
RenderStyle of the scrollbar is calculated (which is multiplied by scale factor)
and the style is set to the RenderScrollbar at the end of the API.When you move
back to updateScrollbarParts, it checks for the scrollbar thickness, if the
thickness is changed it sets the  
box->setChildNeedsLayout(); scrollbarExistenceDidChange(will be invoked if
thickness of the scrollbar is changed) call will relayout if
box->setChildNeedsLayout is set. If Layout is not triggered, Assert 
ASSERT(!rootForThisLayout->needsLayout()); will be invoked in FrameView::layout
function.

+            m_horizontalScrollbar->invalidate();
+        }
     }
 
     if (m_verticalScrollbar) {
@@ -3319,6 +3332,8 @@ void FrameView::updateScrollbarGeometry()
         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);
@@ -3326,6 +3341,17 @@ void FrameView::updateScrollbarGeometry()
         m_verticalScrollbar->offsetDidChange();
         if (m_scrollbarsSuppressed)
             m_verticalScrollbar->setSuppressInvalidation(false);
+
+        if (!m_scrollbarsSuppressed && oldRect != m_verticalScrollbar->frameRect() && m_verticalScrollbar->isCustomScrollbar()) {
+            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 (oldRect.width() != m_verticalScrollbar->frameRect().width())
+                scrollbarExistenceDidChange();
+            m_verticalScrollbar->invalidate();
+        }

[rwlbuis, 2014/10/28 14:59:03]

I see a repeated code pattern, meaning a shared function could be made, but it
probably is not needed for this fix...

     }
 }