source: capability panels as side-stack pages, with a probe for the path

src/source/probe_source.cpp

@@ -128,6 +128,34 @@ class ProbeKind final : public SourceKind {
     f.on_change = [cb](std::function<void()> fn) { *cb = std::move(fn); };
     return f;
   }
+
+  std::vector<SourcePanel> make_panels(DataSource& source) const override {
+    GtkWidget* box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 6);
+    gtk_widget_set_margin_top(box, 12);
+    gtk_widget_set_margin_bottom(box, 12);
+    gtk_widget_set_margin_start(box, 12);
+    gtk_widget_set_margin_end(box, 12);
+    GtkWidget* label = gtk_label_new("Test panel");
+    gtk_widget_set_halign(label, GTK_ALIGN_START);
+    gtk_box_append(GTK_BOX(box), label);
+    // The probe finds the button as the box's last child. It reaches the source only through
+    // `post()`: the command runs inside the next `poll()`, on the producer thread.
+    GtkWidget* poke = gtk_button_new_with_label("Poke");
+    gtk_box_append(GTK_BOX(box), poke);
+    DataSource* src = &source;
+    g_signal_connect_data(
+        poke, "clicked",
+        G_CALLBACK(+[](GtkButton*, gpointer user) {
+          DataSource* s = static_cast<DataSource*>(user);
+          s->post([s] { static_cast<ProbeSource*>(s)->note_applied(); });
+        }),
+        src, nullptr, static_cast<GConnectFlags>(0));
+    SourcePanel p;
+    p.id = "poke";
+    p.title = "Probe";
+    p.widget = box;
+    return {p};
+  }
 };
 
 }  // namespace