Top level windows can be controlled by these flags ：
Qt::MSWindowsFixedSizeDialogHint Gives the window a thin dialog border on
Windows. This style is traditionally used for fixed-size dialogs.
Qt::MSWindowsOwnDC Gives the window its own display context on Windows.
Qt::BypassWindowManagerHint This flag can be used to indicate to the platform
plugin that "all" window manager protocols should be disabled. This flag will
behave different depending on what operating system the application is running
on and what window manager is running. The flag can be used to get a native
window with no configuration set.
Qt::X11BypassWindowManagerHint Bypass the window manager completely. This
results in a borderless window that is not managed at all (i.e. no keyboard
input unless you call QWidget::activateWindow() manually).
Qt::FramelessWindowHint Produces a borderless window. The user cannot move or
resize a borderless window via the window system. On X11, the result of the
flag is dependent on the window manager and its ability to understand Motif
and/or NETWM hints. Most existing modern window managers can handle this.
Qt::NoDropShadowWindowHint Disables window drop shadow on supporting platforms.