The mask could perhaps be an image (where alpha < threshold makes that part of the window transparent) or a Shape or some sort of rendered layer or maybe just any part of the window that is transparent.
Since most window systems do compositing, it's already been possible to have visually transparent windows since Qt 5.0. And since 5.11, a MouseArea or a Pointer Handler can avoid handling events outside a masked shape (using Item.containmentMask). But setting the window mask would allow the mouse and touch events to fall through; and it would also provide visual masking on non-compositing window managers.
This isn't very urgent since it's easy enough to do in C++ already. But we have a number of open bugs about window masking not working correctly on various platforms and in various situations. Making the mask available in QML would exacerbate those.
|For Gerrit Dashboard: QTBUG-71951
|WIP: Add Window.mask to set QWindow::setMask() from QML