Linux下如何让跨进程GTK窗口/控件显示在全屏应用之上？

Got it, let's break this down. The issue you're hitting is that GNOME's window manager (Mutter, in Debian 8's GNOME 3 stack) prioritizes fullscreen windows by default to maintain an immersive experience—so it blocks other windows from drawing on top unless explicitly told otherwise. Here's a C/GTK3 solution that forces your process B's widget window to stay above the fullscreen app A:

First, here's a complete working example that creates a simple GTK button window and ensures it stays atop fullscreen apps:

#include <gtk/gtk.h>
#include <gdk/gdkx.h>
#include <string.h>

int main(int argc, char *argv[]) {
    gtk_init(&argc, &argv);

    // Create the main window (container for your GTK widget)
    GtkWidget *window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title(GTK_WINDOW(window), "Overlay Widget");
    gtk_window_set_default_size(GTK_WINDOW(window), 250, 120);
    gtk_container_set_border_width(GTK_CONTAINER(window), 15);

    // Add your target GTK widget (example uses a button)
    GtkWidget *control = gtk_button_new_with_label("My Overlay Control");
    gtk_container_add(GTK_CONTAINER(window), control);

    // Critical Setting 1: Mark window as "always on top" via GTK API
    gtk_window_set_keep_above(GTK_WINDOW(window), TRUE);

    // Critical Setting 2: Set window type hint to UTILITY (higher priority for overlays)
    gtk_window_set_type_hint(GTK_WINDOW(window), GDK_WINDOW_TYPE_HINT_UTILITY);

    // Critical Setting 3: Force X11 window manager to recognize the "stay on top" state
    // (GTK's wrapper sometimes misses this for GNOME's fullscreen handling)
    GdkWindow *gdk_win = gtk_widget_get_window(window);
    if (gdk_win != NULL) {
        Display *x_display = gdk_x11_window_get_xdisplay(gdk_win);
        Window x_window = gdk_x11_window_get_xid(gdk_win);

        // Fetch X11 atoms for window state properties
        Atom net_wm_state = XInternAtom(x_display, "_NET_WM_STATE", False);
        Atom net_wm_above = XInternAtom(x_display, "_NET_WM_STATE_ABOVE", False);
        Atom net_wm_stay_top = XInternAtom(x_display, "_NET_WM_STATE_STAYS_ON_TOP", False);

        // Craft a client message to send to the window manager
        XEvent x_event;
        memset(&x_event, 0, sizeof(x_event));
        x_event.xclient.type = ClientMessage;
        x_event.xclient.window = x_window;
        x_event.xclient.message_type = net_wm_state;
        x_event.xclient.format = 32;
        x_event.xclient.data.l[0] = 1; // Enable the property
        x_event.xclient.data.l[1] = net_wm_above;
        x_event.xclient.data.l[2] = net_wm_stay_top;
        x_event.xclient.data.l[3] = 0;

        // Send the event to the root window (window manager listens here)
        XSendEvent(x_display, DefaultRootWindow(x_display), False,
                   SubstructureRedirectMask | SubstructureNotifyMask,
                   &x_event);
        XFlush(x_display);
    }

    // Show all widgets and connect cleanup signal
    gtk_widget_show_all(window);
    g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);

    gtk_main();
    return 0;
}

Let's break down why each part matters for GNOME's fullscreen restriction:

• gtk_window_set_keep_above(): GTK's native way to flag a window as top-most, but GNOME's fullscreen logic can override this alone.
• GDK_WINDOW_TYPE_HINT_UTILITY: Tells the window manager this is a small, utility-style window (like a system tray popup), which GNOME allows to bypass fullscreen blocking by default.
• X11 Client Message: Directly communicates with the window manager to set low-level state flags (_NET_WM_STATE_ABOVE and _NET_WM_STATE_STAYS_ON_TOP), ensuring GNOME doesn't ignore the "on top" request.

Compile the code with:

gcc -o overlay_widget overlay_widget.c `pkg-config --cflags --libs gtk+-3.0`

1. Launch your fullscreen process A first.
2. Run the compiled overlay_widget binary—its window should appear directly atop process A.

If the utility window hint doesn't work, try replacing it with GDK_WINDOW_TYPE_HINT_SYSTEM_OVERLAY (even higher priority for system-level overlays). Most Debian 8 GNOME setups allow this for regular user processes.

内容的提问来源于stack exchange，提问作者Jainam MJ