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