Force this window to be treated as the application's main window: destroying
it always quits the message loop (PostQuitMessage), even if other windows
remain.
It defaults to false because, by default, the framework already quits when
the *last* top-level window is destroyed — so a single-window app needs no
configuration, and closing a secondary window never tears the app down. Set
it to true only when one specific window should end the app regardless of
the others.
Force this window to be treated as the application's main window: destroying it always quits the message loop (PostQuitMessage), even if other windows remain.
It defaults to false because, by default, the framework already quits when the *last* top-level window is destroyed — so a single-window app needs no configuration, and closing a secondary window never tears the app down. Set it to true only when one specific window should end the app regardless of the others.