I've created a simple repro application. Just launch it and try to resize its main window. When you've finished resizing it - it paints main window with bugs until you move it. Another issue is that there is a jitter when you resizing the window.
I've attached video demonstrating the issues.

Workaround: add width/height change handlers for the main window and change window's opacity there to force the window repaint. E.g.:

Timer {
        id: invalidateTimer
        interval: 50
        onTriggered: {
            let o = root.opacity;
            root.opacity = o - 0.1;
            root.opacity = o;
        }
    }

    onWidthChanged: invalidateTimer.restart()
    onHeightChanged: invalidateTimer.restart()