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Я так понял зависит от менеджера окон , можно установить опцию Qt::X11BypassWindowManagerHint

я только начинаю в QT работать , подскажите как её применить

QWidget * mainWidget = new QWidget( NULL, Qt::X11BypassWindowManagerHint );

QWidget * window = new QWidget (NULL, Qt::X11BypassWindowMangerHint | Qt::FramelessWindowHint);
window->showFullScreen();

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.

window->show();

window->showFullScreen();