Imagine I have a mainwindow. It has a background image with transparency on the corners, making them curved.

On that is a pictureframe, taking around 2/3 of the area, and while the mainform's background is grayish, the pictureframe's content is currently black - so I can see the edges clearly.

Just below the pictureframe is a QWidget, supposed to align horizontally with it, and touching its lower border.

Now, on WinXP (with classic style enabled, i.e. win2k look), they do align, strangely, when the QWidget is a bit smaller in width, e.g. 700px vs. 704px. How can that be explained?

Then, on Win7 (default style), they do not align, the widget is missing a few rows of pixels left and right, like normally expected.

The mainwindow and pictureframe have only a background image assignment in their stylesheets.
The widget's stylesheet: background-color: rgba(0,0,0,255);

So - what's going on there?