ui: the sidebar is two tabs, and a toggle disables its pane

Replace the sidebar's five-panel scrolling column with an AdwViewStack
carrying two pages — Measure (cursors + measurements) and Signals
(trigger + math + channel list) — with an AdwViewSwitcher above it. The
trigger and cursors toggles change from "reveal/hide my pane" to
"enable/disable my feature, leaving the pane insensitive", following the
disable-not-hide principle. The keys probe loses a duplicate registration
that was toggling the sidebar back on before the exit probe captured its
state; the exit-probe captures now read adw_overlay_split_view_get_show_
sidebar() instead of gtk_widget_get_visible(), which AdwOverlaySplitView
never changes.

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

src/ui_cursors.cpp

@@ -14,7 +14,18 @@ namespace ui_impl {
 
 /// `%.6g s` is exact enough for a cursor readout and short enough for the panel.
 void update_cursors_pane(App& app) {
-  if (app.cursors_body == nullptr || !app.rows->cursors_visible()) return;
+  if (app.cursors_body == nullptr) return;
+  if (!app.rows->cursors_visible()) {
+    // A readout quotes a stored sample; with the instruments off there is no sample it quotes, so
+    // the cells blank rather than keeping the last ones. `"--"` is the same spelling an
+    // unavailable cell already uses -- two spellings of one state in one grid would be the defect.
+    for (const App::CursorLine& line : app.cursors_lines) {
+      if (line.a != nullptr) gtk_label_set_text(GTK_LABEL(line.a), "--");
+      if (line.b != nullptr) gtk_label_set_text(GTK_LABEL(line.b), "--");
+      if (line.d != nullptr) gtk_label_set_text(GTK_LABEL(line.d), "--");
+    }
+    return;
+  }
   if (app.cursors_pane_mode != static_cast<int>(app.rows->cursor_mode()) ||
       app.cursors_pane_rows != app.rows->row_count() ||
       app.cursors_pane_generation != app.model->generation() ||
@@ -117,13 +128,11 @@ void on_cursors_toggled(GtkToggleButton* button, gpointer user) {
   App* app = static_cast<App*>(user);
   const bool on = gtk_toggle_button_get_active(button);
   app->rows->set_cursors_visible(on);
-  // The reset button exists for the cursors: with them hidden there is nothing to
-  // reset, and a button that does nothing reads as broken.
-  gtk_widget_set_visible(app->cursors_reset, on);
-  // The pane is the cursors' readout: with them off there is nothing to read.
-  if (app->cursors_revealer != nullptr) {
-    gtk_revealer_set_reveal_child(GTK_REVEALER(app->cursors_revealer), on);
-  }
+  // Disabled, not hidden, and the same for the reset beside it: with the cursors off there is
+  // nothing to reset, but the button must stay where the user will look for it.
+  gtk_widget_set_sensitive(app->cursors_reset, on);
+  if (app->cursors_panel != nullptr) gtk_widget_set_sensitive(app->cursors_panel, on);
+  if (!on) update_cursors_pane(*app);
   if (on) {
     app->rows->reset_cursors();
     rebuild_cursors_pane(*app);