How do I toggle 'always on top' for a QMainWindow in Qt without causing a flicker or a flash?

Question

void MainWindow::on_actionAlways_on_Top_triggered(bool checked)
{
    Qt::WindowFlags flags = this->windowFlags();
    if (checked)
    {
        this->setWind         


        

Answer 1

Since i recently ran into the same issue:

You can do it by bypassing Qt. For the windows part see @JakePetroules answer.
XCB (X11) version i use:

#ifdef Q_OS_LINUX
#include 
#include 

// Just a simple atom cache helper
xcb_atom_t xcb_get_atom(const char *name){
    if (!QX11Info::isPlatformX11()){
        return XCB_ATOM_NONE;
    }
    auto key = QString(name);
    if(_xcb_atom_cache.contains(key)){
        return _xcb_atom_cache[key];
    }
    xcb_connection_t *connection = QX11Info::connection();
    xcb_intern_atom_cookie_t request = xcb_intern_atom(connection, 1, strlen(name), name);
    xcb_intern_atom_reply_t *reply = xcb_intern_atom_reply(connection, request, NULL);
    if(!reply){
        return XCB_ATOM_NONE;
    }
    xcb_atom_t atom = reply->atom;
    if(atom == XCB_ATOM_NONE){
        DEBUG("Unknown Atom response from XServer: " << name);
    } else {
        _xcb_atom_cache.insert(key, atom);
    }
    free(reply);
    return atom;
}

void xcb_update_prop(bool set, WId window, const char *type, const char *prop, const char *prop2)
{
    auto connection = QX11Info::connection();
    xcb_atom_t type_atom = xcb_get_atom(type);
    xcb_atom_t prop_atom = xcb_get_atom(prop);
    xcb_client_message_event_t event;
    event.response_type = XCB_CLIENT_MESSAGE;
    event.format = 32;
    event.sequence = 0;
    event.window = window;
    event.type = type_atom;
    event.data.data32[0] = set ? 1 : 0;
    event.data.data32[1] = prop_atom;
    event.data.data32[2] = prop2 ? xcb_get_atom(prop2) : 0;
    event.data.data32[3] = 0;
    event.data.data32[4] = 0;

    xcb_send_event(connection, 0, window,
                   XCB_EVENT_MASK_STRUCTURE_NOTIFY | XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT | XCB_EVENT_MASK_PROPERTY_CHANGE,
                   (const char *)&event);
    xcb_flush(connection);
}
#endif

Use like: xcb_update_prop(true, window()->winId(), "_NET_WM_STATE", "_NET_WM_STATE_ABOVE", "_NET_WM_STATE_STAYS_ON_TOP");

Its a bit hackish, but it worked fine on Mate, KDE, GNOME3, XFCE and openbox.