startSystemMove and startSystemResize does not enable aero snap on windows with frameless window.

On Windows startSystemMove and startSystemResize DOES NOT trigger aero snap on frameless windows as suggested by the following blog post:

https://www.qt.io/blog/custom-window-decorations

Note that it's also suggested in https://doc.qt.io/qt-5/qwindow.html#startSystemMove

You could try with the following example:

import QtQuick 2.15
import QtQuick.Controls 2.15
import QtQuick.Window 2.15

ApplicationWindow {
    width: 640
    height: 480
    flags: Qt.FramelessWindowHint | Qt.Window
    visible: true

    background: Rectangle {
       border.color: handler.active ? "green" : "red"
       border.width: 1
    }

    Item {
        anchors.fill: parent

        DragHandler {
            id: handler
            onActiveChanged: if (active) startSystemMove()
        }
    }
}

This is an interresting feature, but since it does not work properly, it feel useless, at least on Windows.

Note that I achived to implements the aero snap feature, and also the  drop shadow, but this needs a bit of Windows specific C++ code to works:

• Needs to react to the WM_NCCALCSIZE event.
• Needs to use the QtWin::extendFrameIntoClientArea.
• Needs to set back some window native flags after settings Qt ones: WS_POPUP | WS_CAPTION | WS_THICKFRAME

So this is not currently possible with pure QML.

It would be nice in fact to have a bit more control over the windows via some flags, something like:

• Qt.SnapWindowHint add the native flags to force snapping support on the windows (in addition with Qt.FramelessWindowHint)

• Qt.DropShadowWindowHint add the native flags to force window shadow (in addition with Qt.FramelessWindowHint) or to allow Qt to emulate it.