ui: read the signal count under the lock in the badge bar; validate the drop payload

src/ui.cpp

@@ -4654,10 +4654,16 @@ void run_app_action(App& app, AppAction action) {
       toggle(app.cursors_toggle);
       break;
     case AppAction::ShowMeasure: dock_show_section(app, "measure"); break;
-    case AppAction::ShowSignals:
+    case AppAction::ShowSignals: {
       // The `+` badge is insensitive without signals; so is its shortcut.
-      if (!app.model->signals().empty()) slideover_toggle(app, "signals");
+      bool have_signals = false;
+      {
+        std::lock_guard<std::mutex> lock(app.model->mutex());
+        have_signals = !app.model->signals().empty();
+      }
+      if (have_signals) slideover_toggle(app, "signals");
       break;
+    }
     case AppAction::LayoutEditor:
       toggle(app.layout_toggle);
       break;