Properly check for fullscreen toggle made through the Window Manager

Fixes #40007.

Co-authored-by: Rémi Verschelde <[email protected]>

platform/linuxbsd/display_server_x11.cpp

@@ -1817,6 +1817,52 @@ bool DisplayServerX11::_window_maximize_check(WindowID p_window, const char *p_a
 	return retval;
 }
 
+bool DisplayServerX11::_window_fullscreen_check(WindowID p_window) const {
+	ERR_FAIL_COND_V(!windows.has(p_window), false);
+	const WindowData &wd = windows[p_window];
+
+	// Using EWMH -- Extended Window Manager Hints
+	Atom property = XInternAtom(x11_display, "_NET_WM_STATE", False);
+	Atom type;
+	int format;
+	unsigned long len;
+	unsigned long remaining;
+	unsigned char *data = nullptr;
+	bool retval = false;
+
+	if (property == None) {
+		return retval;
+	}
+
+	int result = XGetWindowProperty(
+			x11_display,
+			wd.x11_window,
+			property,
+			0,
+			1024,
+			False,
+			XA_ATOM,
+			&type,
+			&format,
+			&len,
+			&remaining,
+			&data);
+
+	if (result == Success) {
+		Atom *atoms = (Atom *)data;
+		Atom wm_fullscreen = XInternAtom(x11_display, "_NET_WM_STATE_FULLSCREEN", False);
+		for (uint64_t i = 0; i < len; i++) {
+			if (atoms[i] == wm_fullscreen) {
+				retval = true;
+				break;
+			}
+		}
+		XFree(data);
+	}
+
+	return retval;
+}
+
 bool DisplayServerX11::window_is_maximize_allowed(WindowID p_window) const {
 	_THREAD_SAFE_METHOD_
 	return _window_maximize_check(p_window, "_NET_WM_ALLOWED_ACTIONS");
@@ -3604,6 +3650,8 @@ void DisplayServerX11::process_events() {
 			case Expose: {
 				DEBUG_LOG_X11("[%u] Expose window=%lu (%u), count='%u' \n", frame, event.xexpose.window, window_id, event.xexpose.count);
 
+				windows[window_id].fullscreen = _window_fullscreen_check(window_id);
+
 				Main::force_redraw();
 			} break;
 

platform/linuxbsd/display_server_x11.h

@@ -265,6 +265,7 @@ class DisplayServerX11 : public DisplayServer {
 
 	void _update_real_mouse_position(const WindowData &wd);
 	bool _window_maximize_check(WindowID p_window, const char *p_atom_name) const;
+	bool _window_fullscreen_check(WindowID p_window) const;
 	void _update_size_hints(WindowID p_window);
 	void _set_wm_fullscreen(WindowID p_window, bool p_enabled);
 	void _set_wm_maximized(WindowID p_window, bool p_enabled);