How to make combobox drop down list transparency?

[ysilent]

Hi All,
This is not a new question, but so far no one can answer this question.
So I guess QT cannot handle this?
What I want to do is first my attached image.
What I'm doing is my second attached image.

My solution is below:
1. Move the drop down list to left a little.
2. Set the drop down list transparency.

For first, I tried all the style sheet, no one works.

For second, I think I can do this but
I worked whole day for this issue, Googled hundreds articles. So I did all basic operations like set Qt::WA_NoSystemBackground for QComboBox, its view, view's palette, view port's palette, but no one works.
For sure I tried style sheet set background to transparent for all widgets, but no one works.
For sure I tried Windows style, motif style, but no one works.

Any one has experience with it?
Or we can say QT cannot do this. Because no solution found on internet.
If QT cannot do this, I cannot understand why people provide QT::transparent or something like this.