Create a Frameless Desktop-Pet-Style Window in Qt

Three key settings for a frameless window

If your window class inherits from QWidget, two lines in the constructor are enough to build the frameless base:

setAttribute(Qt::WA_TranslucentBackground);
setWindowFlags(Qt::SubWindow | Qt::FramelessWindowHint |
               Qt::WindowStaysOnTopHint);

What each parameter does:

• WA_TranslucentBackground: allows a transparent window background, so PNG transparency does not turn black
• FramelessWindowHint: removes the system title bar and border
• WindowStaysOnTopHint: keeps the window always on top

Together, these flags turn your window into a "floating transparent image."

Cross-platform window shape handling

If you want truly "click-through" transparent areas (mouse clicks pass through to the window below), there are two options:

Windows option

this->clearMask();  // do not clip the window shape

Do not actively clip the window shape, so semi-transparent edges are not hard-cut into jagged edges.

Linux option (X11)

#ifdef Q_OS_LINUX
Display *display = XOpenDisplay(nullptr);
if (display) {
    Window window_id = static_cast<Window>(this->winId());

    // build input region from the image alpha channel
    QRegion region(QBitmap::fromImage(image.createAlphaMask()));

    // convert to X11 rectangles and apply
    XRectangle *xrects = new XRectangle[region.rectCount()];
    // ... fill rectangle data ...

    XShapeCombineRectangles(display, window_id, ShapeInput, 0, 0,
                           xrects, count, ShapeSet, YXBanded);

    delete[] xrects;
    XCloseDisplay(display);
}
#endif

This solution uses the X11 Shape extension to clip the input region at the system level, which is more thorough than handling it in Qt.