nwg-panel: fix double-bar problem on crash

hosts/common/programs/nwg-panel/default.nix

@@ -147,6 +147,7 @@ in
     suggestedPrograms = [
       "brightnessctl"
       "pactl"  # pactl required by `per-app-volume` component.
+      "sane-die-with-parent"
     ] ++ lib.optionals (cfg.config.torch != null) [
       "torch-toggle"
     ];
@@ -212,7 +213,8 @@ in
       # to debug styling, run with GTK_DEBUG=interactive
       # N.B.: G_MESSAGES_DEBUG=all causes the swaync icon to not render
       # command = "env G_MESSAGES_DEBUG=all nwg-panel";
-      command = "nwg-panel";
+      # XXX: try `nwg-panel & ; kill $$`. the inner nwg-panel doesn't die (without sane-die-with-parent), and hence the service would be prone to maintaining _multiple_ bars.
+      command = "sane-die-with-parent --descendants --signal SIGKILL nwg-panel";
     };
   };
 }