Bug: White Bar Appears on Top Window with System Border Enabled

[mkbkyyds]

### Description:
With the system border enabled via CMake and a QML window configured as a top window using flags, switching window focus causes a white bar to appear at the top briefly before disappearing.

### Reproduction Steps:

1. Build with system border enabled in CMake.
2. Launch a QML window configured as a top window using flags.
3. Switch focus from the QML window to another window, then switch back.
4. The white bar appears at the top momentarily.

[wangwenx190]

Qt5 bug, may not be easily fixable

[tredy6t]

` Component.onCompleted: {
windowAgent.setup(window)
window.visible = true
windowAgent.centralize()

    window.width = 1080
    window.height = 720
    fixTopBorderTimer.start()
}

Timer {
    id: fixTopBorderTimer
    interval: 100
    repeat: false
    onTriggered: {
        console.log("resize triggered")
        // add 1px
        window.width += 1
        // restore
        window.width -= 1
        console.log("Forced resize executed")

        window.minimumHeight= 720
        window.maximumHeight= 720
        window.minimumWidth= 1080
        window.maximumWidth= 1080
    }
}

`
It works by resize window while window setuped