Abnormal case in scroll view

[qzhang8]

BUG DETAILED DESCRIPTIONS
===========================================================
In scroll widget, blank area of browser will appear following steps below:
1 left-press on mouse in scroll view area and do not release mouse
2 move mouse to the bottom of browser

Another abnormal case for scroll widget is: 
1. Left press on mouse in scroll view area and do not release mouse
2. Move mouse out of scroll view area and release mouse
3. Move mouse into scroll view area again
you will find that just moving mouse will scroll the view