How to make a window always on top?

compiz, qt5, ubuntu, window-managers, x11

Solution

the EWMH specification explicitly states that:

_NET_WM_STATE_ABOVE and _NET_WM_STATE_BELOW are mainly meant for user preferences and should not be used by applications e.g. for drawing attention to their dialogs (the Urgency hint should be used in that case, see the section called “Urgency”).

so window managers have no responsibility to respect applications which set this property directly (i.e by XChangeProperty) by themselves. this property can be changed only by sending a client message to the root window which window managers listens on.

i don't know how to do it in high-level gui toolkits like Qt, but here is how to do it in plain X11.(see EWMH spec, or _wnck_change_state for a sample implementation).

//file: test.c
//to build it, run
//shell> gcc test.c -lX11

#include <X11/Xlib.h>   

#define _NET_WM_STATE_REMOVE        0    /* remove/unset property */
#define _NET_WM_STATE_ADD           1    /* add/set property */
#define _NET_WM_STATE_TOGGLE        2    /* toggle property  */


// change a window's _NET_WM_STATE property so that it can be kept on top.
// @display: x11 display singleton.
// @xid    : the window to set on top.
Status x11_window_set_on_top (Display* display, Window xid)
{
    XEvent event;
    event.xclient.type = ClientMessage;
    event.xclient.serial = 0;
    event.xclient.send_event = True;
    event.xclient.display = display;
    event.xclient.window  = xid;
    event.xclient.message_type = XInternAtom (display, "_NET_WM_STATE", False);
    event.xclient.format = 32;

    event.xclient.data.l[0] = _NET_WM_STATE_ADD;
    event.xclient.data.l[1] = XInternAtom (display, "_NET_WM_STATE_ABOVE", False);
    event.xclient.data.l[2] = 0; //unused.
    event.xclient.data.l[3] = 0;
    event.xclient.data.l[4] = 0;

    return XSendEvent (display, DefaultRootWindow(display), False,
                       SubstructureRedirectMask|SubstructureNotifyMask, &event);
}

// a sample main function for testing.
// shell> ./a.out window_xid
int main (int argc, char** argv)
{
    Window xid = strtol (argv[1], NULL, 0); 
    Display* display = XOpenDisplay (NULL);

    x11_window_set_on_top (display, xid);

    XFlush (display); //for simplicity, no event loops here.

    XCloseDisplay (display);
}

also note that in some x11 environment(e.g. compiz), system menus are provided by a separate decorator program instead of the compositing window manager.

Problem

Original source