xwayland: set focus to XCB_POINTER_ROOT rather than XCB_NONE

Fixes an issue with keyboard grabs not working when no XWayland windows
are focused.

xwayland/xwm.c

@@ -363,9 +363,13 @@ static void xwm_focus_window(struct wlr_xwm *xwm,
 	// We handle cases where focus_surface == xsurface because we
 	// want to be able to deny FocusIn events.
 	if (!xsurface) {
+		// XCB_POINTER_ROOT is described in xcb documentation but isn't
+		// actually defined in the headers. It's distinct from XCB_NONE
+		// (which disables keyboard input entirely and causes issues
+		// with keyboard grabs for e.g. popups).
 		xcb_set_input_focus_checked(xwm->xcb_conn,
 			XCB_INPUT_FOCUS_POINTER_ROOT,
-			XCB_NONE, XCB_CURRENT_TIME);
+			1L /*XCB_POINTER_ROOT*/, XCB_CURRENT_TIME);
 		return;
 	}