ui: the cursors and measure pages are insensitive until the stream has signals

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui_probe.cpp

@@ -192,9 +192,17 @@ gboolean exit_probe_step3b(gpointer user) {
          c = gtk_widget_get_next_sibling(c)) {
       ++children;
     }
-    std::fprintf(stderr, "[readouts] pages cursors=%d measure=%d dock_children=%d dock_w=%d\n",
+    // Both pages follow the dock's availability: insensitive without signals, sensitive with them.
+    a->dbg_pages_sensitive = a->cursors_panel != nullptr && a->measures_panel != nullptr &&
+                                     gtk_widget_get_sensitive(a->cursors_panel) &&
+                                     gtk_widget_get_sensitive(a->measures_panel)
+                                 ? 1 : 0;
+    std::fprintf(stderr,
+                 "[readouts] pages cursors=%d measure=%d dock_children=%d dock_w=%d "
+                 "pages_sensitive=%d signals=%d\n",
                  slideover_has_page(*a, "cursors") ? 1 : 0, slideover_has_page(*a, "measure") ? 1 : 0,
-                 children, a->dbg_side_w);
+                 children, a->dbg_side_w, a->dbg_pages_sensitive,
+                 a->model->signals().empty() ? 0 : 1);
   }
   a->dbg_dock_scrollers = 0;
   for (GtkWidget* c = gtk_widget_get_first_child(a->side_box); c != nullptr;