How to build a transparent ScrollArea widget?

[myfifth]

I'd like to build my own ScrollArea widget, which would be transparent if it overlaps other widgets or texts, based on QScrollArea.

Basically I set WA_NoSystemBackground attribute on both ScrollArea and its content widget, but the resulting appearance was not exactly expected.
1. the entire ScrollArea displayed a black background, I've no idea why it's black or it just meant that black represented blank background. What I expected was that it would display everything intact.
2. the button under ScrollArea can be viewed and can't respond to mouse click, this demonstrated the transparency effect partially worked, but the button was gone away while scrolling and never got repainted.
3. I'm afraid there is some extra paintings supposed to be handled in the case, but how?

I've built a quick demo to show my problem, hopefully someone could give me some reference. thx.

QTransparentScrollArea.zip