[Qt-interest] "white block" on QScrollBar's background

[James Yan]

Hey, All.
  I try to set background of a scrollbar(belong to a scrollarea) to pure black, so i work with setStyleSheet like following:

===
scrollArea->verticalScrollBar()->setStyleSheet("QScrollBar:vertical{background: black; }");
===

but, the result always shows up with white block(looks 1X1 pixel for each) on the pure black, make the background looks like chess board. What's wrong with it? any suggestion? 

btw,i use Qt on Mac.

thanks.
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