setting border-radius does not clip the background?

[Jakob]

Running Qt5.4.0 64bit on Windows 7.

I set the following stylesheet on one of my panels:

*[styleAsPanel="true"]
{
    background-color: white;
    border: 1px solid #4a4a4a;
    border-top-left-radius: 0;
    border-top-right-radius: 0;
    border-bottom-right-radius: 4px;
    border-bottom-left-radius: 4px;
}

Expectation: the panel background is clipped by setting the border radius (as documented also on http://doc.qt.io/qt-5/stylesheet-reference.html#border-radius-prop)

Reality: background is not clipped, see http://ctrlv.in/675860
[edit: I created a snapshot of a bottom-right corner - the problem however appears on each of the corners]

Am I missing something, or did I hit a bug?

[Jakob]

I did some additional investigation and as far as I can tell, this really looks like a bug report. I filed https://bugreports.qt.io/browse/QTBUG-49965 for it

[Chris Kawa]

It's more of a limitation than a bug I think.

To OS, by default a window is a rectangle, no matter how you paint it, and stylesheets are just a way to paint it.
To cut out the corners you would need a mask. Something like:

auto frame = new QWidget(parent, Qt::Popup);
frame->setStyleSheet("background-color: red; border: 1px solid green; border-radius: 6px;");

QPainterPath path;
path.addRoundedRect(frame->rect(), 6, 6);
frame->setMask(path.toFillPolygon().toPolygon());

frame->show();

The problem is that the mask is a bitmap (0-1), not an image with alpha, so you get a jagged edges, but for some radii it works pretty well.