send XDG fullscreen state to Wayland clients

Add client_set_fullscreen backend vtable entry. Call
wlr_xdg_toplevel_set_fullscreen on enter/leave so clients
(e.g. Firefox) know to render content at full size.

src/actions.c

@@ -1033,6 +1033,9 @@ void wFullscreenWindow(WWindow *wwin)
 
 	wwin->flags.fullscreen = True;
 
+	if (wm_backend->client_set_fullscreen)
+		wm_backend->client_set_fullscreen(wwin->client_win, 1);
+
 	wWindowConfigureBorders(wwin);
 
 	ChangeStackingLevel(wwin->frame->core, WMFullscreenLevel);
@@ -1059,6 +1062,9 @@ void wUnfullscreenWindow(WWindow *wwin)
 
 	wwin->flags.fullscreen = False;
 
+	if (wm_backend->client_set_fullscreen)
+		wm_backend->client_set_fullscreen(wwin->client_win, 0);
+
 	if (wwin->frame->core->stacking->window_level == WMFullscreenLevel) {
 		if (WFLAGP(wwin, sunken)) {
 			ChangeStackingLevel(wwin->frame->core, WMSunkenLevel);

src/backend/backend.h

@@ -1540,6 +1540,7 @@ typedef struct WMBackend {
      * X11: no-op (handled via _NET_WM_STATE property).
      * Wayland: sends xdg_toplevel set_maximized/set_fullscreen. */
     void (*client_set_maximized)(WNativeWindow win, int maximized);
+    void (*client_set_fullscreen)(WNativeWindow win, int fullscreen);
 
     /*
      * Send a WM_PROTOCOLS message to a client (e.g. WM_DELETE_WINDOW).

src/backend/wayland/wl_client.c

@@ -568,6 +568,14 @@ void wl_client_set_maximized(WNativeWindow win, int maximized)
         wlr_xdg_toplevel_set_maximized(v->xdg_surface->toplevel, maximized);
 }
 
+void wl_client_set_fullscreen(WNativeWindow win, int fullscreen)
+{
+    struct wl_toplevel_view *v = wl_find_view_by_id(win);
+    if (!v) return;
+    if (v->xdg_surface && v->xdg_surface->toplevel)
+        wlr_xdg_toplevel_set_fullscreen(v->xdg_surface->toplevel, fullscreen);
+}
+
 void wl_client_send_protocol(WWindow *wwin, unsigned long protocol,
                                     unsigned long timestamp)
 {

src/backend/wayland/wl_stubs.c

@@ -48,6 +48,7 @@ void *wl_create_rimage_from_drawable(void *rcontext, WMDrawable drawable);
 void wl_icon_update_pixmap(WIcon *icon);
 void wl_client_set_state(WWindow *wwin, int state, WNativeWindow icon_win);
 void wl_client_set_maximized(WNativeWindow win, int maximized);
+void wl_client_set_fullscreen(WNativeWindow win, int fullscreen);
 void wl_client_send_protocol(WWindow *wwin, unsigned long protocol, unsigned long timestamp);
 void wl_client_kill_impl(WWindow *wwin);
 char *wl_get_command_for_window(WNativeWindow win, int elements);
@@ -462,6 +463,7 @@ const WMBackend wl_backend = {
     .client_restore         = wl_noop_client_restore,
     .client_set_state       = wl_client_set_state,
     .client_set_maximized   = wl_client_set_maximized,
+    .client_set_fullscreen  = wl_client_set_fullscreen,
     .client_send_protocol   = wl_client_send_protocol,
     .client_kill            = wl_client_kill_impl,
     .client_get_colormap_windows = wl_noop_client_get_colormap_windows,

src/backend/x11/x11_backend.c

@@ -4906,6 +4906,7 @@ const WMBackend x11_backend = {
     .client_restore          = x11_client_restore,
     .client_set_state        = x11_client_set_state,
     .client_set_maximized    = NULL, /* X11: handled via _NET_WM_STATE property */
+    .client_set_fullscreen   = NULL, /* X11: handled via _NET_WM_STATE property */
     .client_send_protocol    = x11_client_send_protocol,
     .client_kill             = x11_client_kill,
     .client_get_colormap_windows = x11_client_get_colormap_windows,