ui: each pane carries its own switch, the same one the header bar has

The trigger pane and the cursors pane now have a `GtkSwitch` in their heading, and it is *one control in
two places* with the header bar's button: both run the same action (`run_app_action()`), and
`sync_panel_switches()` re-reads each pair from the toolbar's state after any change, so clicking either
end moves both. A pane whose feature is off is disabled, which is exactly the state you need the switch
in, so it belongs *in* the pane rather than only in the toolbar.

Three things it needed:

- **The sensitive target moved to the pane's body.** A control inside an insensitive parent cannot be
  clicked, so a switch that greyed out with its own pane could never turn it back on. `on_trigger_toggled`
  and `on_cursors_toggled` now disable `trigger_body` / `cursors_body`, and so does the startup default;
  the headings, with their switches, stay live.
- **The trigger's heading moved out of `trigger-panel.ui`** and is built in `build_trigger_panel()`,
  which returns the heading row and the panel together. It has to be a sibling of the body for the switch
  to stay usable, and keeping it in the file would have meant the file owning a widget the code must
  arrange. The file's own panel is unchanged otherwise, so the `[trigger panel:]` counts still read.
- **A guard on the sync.** Setting a `GtkSwitch` emits `notify::active` for a programmatic write just as
  it does for a click, so without `App::panel_switch_sync` the program's own write would come back as a
  click and flip the feature again.

Readings, both directions in one line:

    trigger panel sensitive before the probe toggled it: 0; … ; pane switches live while their panes are off=1
    panel switches: trigger=1/1 cursors=1/1 drove_toolbar=1  (pane switch / header bar button)

`pane switches live …=1` is the design's point and is read before anything turns either pane on; `0` would
be a switch drawn, greyed and useless. The trigger pair is moved from the *toolbar* end and the cursors
pair from the *pane* end, so the line carries both halves of the sync, and `drove_toolbar=1` is the pane's
own switch having turned the cursors on with the button following. `trigger panel sensitive …=0` is the
body, not the panel.

Verified: the two lines above; `[side-tab-probe] … advanced=4`; `make test-ui` 14 files; zero
`Gtk-CRITICAL` and no missing icon. Docs: the new reading in `probe-readings.md` with what a `0` means,
the invariants bullet for the one-control-in-two-places rule and its two traps, the sidebar bullet's
sensitive target, and the README note on why the switch is in the pane.

src/ui.cpp

@@ -4355,6 +4355,39 @@ void on_window_popover_press(GtkGestureClick*, int, double x, double y, gpointer
 /// gtk_toggle_button_set_active(), which emits ::toggled and so runs exactly the handler a
 /// click would; the non-toggles call their click handlers with a null button, which they
 /// already ignore. Split out of the key handler so the probe can fire a row directly.
+/// The pane's switches and the header bar's buttons are one control with two places to click, so a
+/// change in either re-reads the other. `panel_switch_sync` is the guard: setting a `GtkSwitch` emits
+/// its own `notify::active`, and without it the program's write would come back as a click.
+void sync_panel_switches(App& app) {
+  if (app.panel_switch_sync) return;
+  app.panel_switch_sync = true;
+  const auto mirror = [](GtkWidget* panel_switch, GtkWidget* toolbar_toggle) {
+    if (panel_switch == nullptr || toolbar_toggle == nullptr) return;
+    gtk_switch_set_active(GTK_SWITCH(panel_switch),
+                          gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(toolbar_toggle)) == TRUE);
+  };
+  mirror(app.trigger_switch, app.trigger_toggle);
+  mirror(app.cursors_switch, app.cursors_toggle);
+  app.panel_switch_sync = false;
+}
+
+/// A pane's own switch drives the *same action* as the header bar's button, so either place reaches
+/// the feature down one path, and the sync above is what keeps the two showing the same thing.
+void on_panel_switch_toggled(GObject* obj, GParamSpec*, gpointer user) {
+  App* app = static_cast<App*>(user);
+  if (app->panel_switch_sync) return;
+  const bool on = gtk_switch_get_active(GTK_SWITCH(obj)) != FALSE;
+  const bool is_trigger = app->trigger_switch != nullptr && obj == G_OBJECT(app->trigger_switch);
+  GtkWidget* toggle = is_trigger ? app->trigger_toggle : app->cursors_toggle;
+  if (toggle == nullptr) return;
+  // The action toggles the toolbar button, so it is run only when the two actually disagree -- which
+  // is what makes this a *set* rather than a second toggle of something already in that state.
+  if ((gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(toggle)) != FALSE) != on) {
+    run_app_action(*app, is_trigger ? AppAction::TriggerPanel : AppAction::CursorsPane);
+  }
+  sync_panel_switches(*app);
+}
+
 void run_app_action(App& app, AppAction action) {
   const auto toggle = [](GtkWidget* button) {
     if (button == nullptr) return;
@@ -4771,10 +4804,12 @@ void on_activate(GtkApplication* application, gpointer user) {
   app->signal_panel = build_signal_panel(*app);
   gtk_box_append(GTK_BOX(signals_page), app->signal_panel);
 
-  // Both instruments start off, so both panels start insensitive -- stated here by the one place
-  // that knows the default, rather than left to whichever widget happened to be built first.
-  gtk_widget_set_sensitive(app->trigger_panel, FALSE);
-  gtk_widget_set_sensitive(app->cursors_panel, FALSE);
+  // Both instruments start off, so both *bodies* start insensitive -- stated here by the one place
+  // that knows the default, rather than left to whichever widget happened to be built first. The
+  // bodies and not the panels: each panel's heading carries the switch that turns it on, and a
+  // control inside an insensitive parent cannot be clicked.
+  gtk_widget_set_sensitive(app->trigger_body, FALSE);
+  gtk_widget_set_sensitive(app->cursors_body, FALSE);
   gtk_widget_set_sensitive(app->cursors_reset, FALSE);
 
   if (app->side_stack != nullptr) {
@@ -5685,6 +5720,12 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 app.dbg_trigger_sensitive, app.dbg_switcher_visible,
                 rows.cursors_visible() ? 1 : 0, rows.time_window(),
                 app.window_limit_s);
+    std::printf("[udpscope]   panel switches: trigger=%d/%d cursors=%d/%d drove_toolbar=%d "
+                "(pane switch / header bar button; `drove_toolbar` is the pane's own switch having "
+                "moved the button)\n",
+                app.dbg_panel_switch_trigger, app.dbg_toolbar_switch_trigger,
+                app.dbg_panel_switch_cursors, app.dbg_toolbar_switch_cursors,
+                app.dbg_panel_drove_toolbar);
     std::printf("[udpscope]   switcher: policy=%s pills=%d labels=%d names=%d icon=%dx%d "
                 "size=%dx%d\n",
                 app.dbg_switcher_policy == 0 ? "narrow" : (app.dbg_switcher_policy == 1 ? "wide" : "?"),
@@ -5855,9 +5896,11 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 app.dbg_badge_frames[0], app.dbg_badge_frames[1], app.dbg_badge_frames[2],
                 app.dbg_badge_frames[3]);
     std::printf("[udpscope]   trigger panel sensitive before the probe toggled it: %d; "
-                "signal list items=%d selected=%d; column visible after sidebar toggle off=%d\n",
+                "signal list items=%d selected=%d; column visible after sidebar toggle off=%d; "
+                "pane switches live while their panes are off=%d\n",
                 app.dbg_trigger_sensitive_default, app.dbg_trigger_items,
-                app.dbg_trigger_selected, app.dbg_side_column_after_hide);
+                app.dbg_trigger_selected, app.dbg_side_column_after_hide,
+                app.dbg_panel_switch_live);
     {
       // The panel's *frame* read back: the row boxes the `.ui` holds, and that every control the code built
       // ended up inside the panel rather than in a row that is not there. A control is created in code and

src/ui/cursors-panel.ui

@@ -38,6 +38,15 @@
             <property name="css-classes">flat</property>
           </object>
         </child>
+        <child>
+          <!-- The pane's own switch, the same one the header bar carries: a feature whose pane is
+               disabled while it is off needs its control *in* the pane, or turning it back on means
+               finding the toolbar button that greyed it out. `body` is what goes insensitive, not the
+               panel, so this stays live. -->
+          <object class="GtkSwitch" id="cursors_switch">
+            <property name="valign">center</property>
+          </object>
+        </child>
       </object>
     </child>
     <child>

src/ui/trigger-panel.ui

@@ -2,8 +2,10 @@
 <!-- SPDX-License-Identifier: MIT -->
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
 <!--
-  The trigger panel's frame: the heading, one labelled row per setting, the two buttons that run it, and
-  the search block.
+  The trigger panel's frame: one labelled row per setting, the two buttons that run it, and the search
+  block. Its *heading* is not here: the heading row carries the switch that turns trigger mode on and
+  off, and this panel's body is what goes insensitive when it is off, so the heading has to be its
+  sibling rather than a child of it (`build_trigger_panel()` returns the two together).
 
   Every row here is a label and a place for a control, and the control itself is built in code. A
   dropdown's model and a spin button's range are the *application's* (the condition list, the level
@@ -24,12 +26,6 @@
     <property name="margin-bottom">8</property>
     <property name="margin-start">8</property>
     <property name="margin-end">8</property>
-    <child>
-      <object class="GtkLabel" id="trigger_heading">
-        <property name="label" translatable="yes">Trigger</property>
-        <property name="halign">start</property>
-      </object>
-    </child>
     <child>
       <object class="GtkBox" id="signal_row">
         <property name="orientation">horizontal</property>

src/ui_app.hpp

@@ -227,6 +227,18 @@ struct App {
   /// is what it costs.
   int dbg_switcher_w = -1;
   int dbg_switcher_h = -1;
+  /// The panes' own switches and the header bar's buttons, read together by the exit probe: the two
+  /// must agree, and `dbg_panel_drove_toolbar` is the pane's switch having moved the button.
+  int dbg_panel_switch_trigger = -1;
+  int dbg_toolbar_switch_trigger = -1;
+  int dbg_panel_switch_cursors = -1;
+  int dbg_toolbar_switch_cursors = -1;
+  int dbg_panel_drove_toolbar = -1;
+  /// Both panes' switches being *sensitive* while their panes are off -- read before anything turns
+  /// one on. It is the whole point of putting them in the heading: a control inside an insensitive
+  /// parent cannot be clicked, so a switch that went insensitive with its own pane could never turn
+  /// it back on.
+  int dbg_panel_switch_live = -1;
   int dbg_switcher_policy = -1;
   /// Whether `name_side_tabs()` has named the pills: it can only run once the switcher is mapped.
   bool side_tabs_named = false;
@@ -236,6 +248,16 @@ struct App {
   GtkWidget* cursors_reset = nullptr;
   /// The cursors panel's root, kept because cursors-off disables it rather than hiding it.
   GtkWidget* cursors_panel = nullptr;
+  /// The panes' *own* switches, in their header rows, and the panel bodies those switches disable.
+  /// The switch and the header bar's button are one control with two places to click: each runs the
+  /// same action, and `sync_panel_switches()` re-reads both from the toolbar's state after any
+  /// change, so they cannot disagree.
+  GtkWidget* trigger_switch = nullptr;
+  GtkWidget* cursors_switch = nullptr;
+  GtkWidget* trigger_body = nullptr;
+  /// True while `sync_panel_switches()` is writing the switches: setting a `GtkSwitch` emits its own
+  /// `notify::active`, and without this the program's own write would come back as a click.
+  bool panel_switch_sync = false;
   /// The cursors pane: listing what the instruments read -- T0/T1/dT always, per row an Y1/Y2/dY
   /// where rulers are up, and in `mix` mode the value each cursor crosses each trace at.
   GtkWidget* cursors_mode_drop = nullptr;
@@ -1388,6 +1410,10 @@ void on_right_click(GtkGestureClick* gesture, int, double x, double y, gpointer
 gboolean on_drop_channel(GtkDropTarget*, const GValue* value, double x, double y, gpointer user);
 void on_side_tab_changed(GObject*, GParamSpec*, gpointer user);
 void select_side_tab(App& app, const char* name);
+/// Reads each pane's own switch back off the header bar's button, and the other way round: the two are
+/// one control in two places, and `panel_switch_sync` keeps the write from looking like a click.
+void sync_panel_switches(App& app);
+void on_panel_switch_toggled(GObject* obj, GParamSpec*, gpointer user);
 void update_side_visibility(App& app);
 void on_sidebar_toggled(GtkToggleButton* button, gpointer user);
 void rebuild_signal_panels(App& app);

src/ui_cursors.cpp

@@ -96,6 +96,12 @@ GtkWidget* build_cursors_panel(App& app) {
   app.cursors_mode_drop = GTK_WIDGET(gtk_builder_get_object(ui, "mode_drop"));
   app.cursors_reset = GTK_WIDGET(gtk_builder_get_object(ui, "reset"));
   app.cursors_body = GTK_WIDGET(gtk_builder_get_object(ui, "body"));
+  app.cursors_switch = GTK_WIDGET(gtk_builder_get_object(ui, "cursors_switch"));
+  if (app.cursors_switch != nullptr) {
+    gtk_widget_set_tooltip_text(app.cursors_switch,
+                                "the cursors, the same switch the header bar carries");
+    g_signal_connect(app.cursors_switch, "notify::active", G_CALLBACK(on_panel_switch_toggled), &app);
+  }
   if (box == nullptr || app.cursors_mode_drop == nullptr || app.cursors_reset == nullptr ||
       app.cursors_body == nullptr) {
     // A missing id is a mistake in the `.ui`. The panel comes up empty rather than taking the window
@@ -129,15 +135,19 @@ void on_cursors_toggled(GtkToggleButton* button, gpointer user) {
   const bool on = gtk_toggle_button_get_active(button);
   app->rows->set_cursors_visible(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.
+  // nothing to reset, but the button must stay where the user will look for it. The *body* goes
+  // insensitive and not the panel, because the panel's header carries the switch that turns the
+  // cursors back on -- and a control inside an insensitive parent cannot be clicked.
   gtk_widget_set_sensitive(app->cursors_reset, on);
-  if (app->cursors_panel != nullptr) gtk_widget_set_sensitive(app->cursors_panel, on);
+  if (app->cursors_body != nullptr) gtk_widget_set_sensitive(app->cursors_body, on);
   if (!on) update_cursors_pane(*app);
   if (on) {
     app->rows->reset_cursors();
     rebuild_cursors_pane(*app);
     update_cursors_pane(*app);
   }
+  // The pane's own switch shows the same state, whichever of the two was clicked.
+  sync_panel_switches(*app);
 }
 
 void on_cursors_reset(GtkButton*, gpointer user) {

src/ui_probe.cpp

@@ -85,6 +85,28 @@ gboolean exit_probe_step3(gpointer user) {
     a->dbg_switcher_w = gtk_widget_get_width(a->side_switcher);
     a->dbg_switcher_h = gtk_widget_get_height(a->side_switcher);
   }
+  // The panes' own switches, and *both* directions of the sync: the header bar's trigger button is
+  // flipped here and the pane's switch has to follow, then the cursors' switch is driven from the
+  // pane and the button is what has to follow. Both are read back after the fact, so the two pairs in
+  // the dump are the claim: they agree, having been moved from opposite ends.
+  if (a->trigger_toggle != nullptr) {
+    gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(a->trigger_toggle), TRUE);
+  }
+  a->dbg_panel_switch_trigger =
+      a->trigger_switch != nullptr && gtk_switch_get_active(GTK_SWITCH(a->trigger_switch)) ? 1 : 0;
+  a->dbg_toolbar_switch_trigger =
+      a->trigger_toggle != nullptr &&
+              gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(a->trigger_toggle))
+          ? 1 : 0;
+  if (a->cursors_switch != nullptr) {
+    gtk_switch_set_active(GTK_SWITCH(a->cursors_switch), TRUE);
+    a->dbg_panel_switch_cursors = gtk_switch_get_active(GTK_SWITCH(a->cursors_switch)) ? 1 : 0;
+    a->dbg_toolbar_switch_cursors =
+        a->cursors_toggle != nullptr &&
+                gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(a->cursors_toggle))
+            ? 1 : 0;
+    a->dbg_panel_drove_toolbar = a->dbg_toolbar_switch_cursors;
+  }
   for (GtkWidget* pill = a->side_switcher != nullptr ? gtk_widget_get_first_child(a->side_switcher)
                                                     : nullptr;
        pill != nullptr; pill = gtk_widget_get_next_sibling(pill)) {
@@ -116,7 +138,7 @@ gboolean exit_probe_step3(gpointer user) {
   const int on_signals = page != nullptr && std::strcmp(page, "signals") == 0 ? 1 : 0;
   a->dbg_signal_page_current =
       on_signals && gtk_widget_get_first_child(a->signal_list) != nullptr ? 1 : 0;
-  a->dbg_trigger_sensitive = on_signals && gtk_widget_get_sensitive(a->trigger_panel) ? 1 : 0;
+  a->dbg_trigger_sensitive = on_signals && gtk_widget_get_sensitive(a->trigger_body) ? 1 : 0;
   // `gtk_widget_get_visible` on the sidebar's GtkBox is always 1: AdwOverlaySplitView hides its
   // sidebar child via `gtk_widget_set_child_visible()`, not `gtk_widget_set_visible()`. The useful
   // reading is whether the split view is currently showing the sidebar panel.
@@ -2299,7 +2321,11 @@ gboolean export_probe(gpointer user) {
 gboolean exit_probe_step1(gpointer user) {
   App* a = static_cast<App*>(user);
   // Record the panel states before the toggles are exercised.
-  a->dbg_trigger_sensitive_default = gtk_widget_get_sensitive(a->trigger_panel) ? 1 : 0;
+  a->dbg_trigger_sensitive_default = gtk_widget_get_sensitive(a->trigger_body) ? 1 : 0;
+  a->dbg_panel_switch_live =
+      a->trigger_switch != nullptr && gtk_widget_get_sensitive(a->trigger_switch) &&
+              a->cursors_switch != nullptr && gtk_widget_get_sensitive(a->cursors_switch)
+          ? 1 : 0;
   if (a->trigger_toggle != nullptr) {
     gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(a->trigger_toggle), TRUE);
   }
@@ -2342,7 +2368,7 @@ void cursors_probe_report(App& a, const char* what) {
                "[cursors-probe] %-14s mode=%-10s on=%d pane_sensitive=%d t0=%.6f t1=%.6f dt=%.6f "
                "T=[%s] lines=%d marks=%d [%s]\n",
                what, kModes[mode], a.rows->cursors_visible() ? 1 : 0,
-               a.cursors_panel != nullptr && gtk_widget_get_sensitive(a.cursors_panel) ? 1 : 0,
+               a.cursors_body != nullptr && gtk_widget_get_sensitive(a.cursors_body) ? 1 : 0,
                a.rows->cursor_a_x(), a.rows->cursor_b_x(),
                a.rows->cursor_b_x() - a.rows->cursor_a_x(), t_text.c_str(), n,
                a.rows->crossings_visible(), lines.c_str());

src/ui_trigger.cpp

@@ -196,14 +196,18 @@ void on_trigger_toggled(GtkToggleButton* button, gpointer user) {
     app->model->set_signal_enabled(static_cast<std::size_t>(signal), true);
   }
   // Disabled, not hidden: a panel you can still see is a panel you can read, and the settings you
-  // are about to switch back on must not vanish while you look for them.
-  gtk_widget_set_sensitive(app->trigger_panel, on);
+  // are about to switch back on must not vanish while you look for them. The *body* is what goes
+  // insensitive and not the panel, because the panel's heading row carries the switch that turns
+  // this back on -- and a control inside an insensitive parent cannot be clicked.
+  gtk_widget_set_sensitive(app->trigger_body, on);
   // The event-history button is part of trigger mode and the pane it opens is *on demand*; leaving
   // trigger mode takes the button away and collapses the pane, so going back to streaming is not a
   // column of stale events beside the plot. Recording is the other half of that answer, so this is
   // `sync_events_availability()`'s call and not the trigger's alone.
   sync_events_availability(*app);
   gtk_button_set_label(GTK_BUTTON(app->arm_button), on ? "Disarm" : "Arm");
+  // The pane's own switch shows the same state, whichever of the two was clicked.
+  sync_panel_switches(*app);
 }
 
 /// The way out of trigger mode, from inside the panel that froze the view: it is
@@ -236,6 +240,8 @@ GtkWidget* build_trigger_panel(App& app) {
   GtkBuilder* ui = ui_load(app, "/org/udpscope/ui/trigger-panel.ui");
   GtkWidget* box = GTK_WIDGET(gtk_builder_get_object(ui, "trigger_panel"));
   if (box == nullptr) return gtk_box_new(GTK_ORIENTATION_VERTICAL, 6);
+  // The *body* is what trigger-off disables; this panel's index into it is `trigger_body`.
+  app.trigger_body = box;
 
   app.trigger_signal_drop = gtk_drop_down_new(G_LIST_MODEL(gtk_string_list_new(nullptr)), nullptr);
   g_signal_connect(app.trigger_signal_drop, "notify::selected",
@@ -422,7 +428,29 @@ GtkWidget* build_trigger_panel(App& app) {
                      &app);
   }
   sync_trigger_rows(app);
-  return box;
+
+  // The heading row, and the switch that is the *same* control as the header bar's button. Built here
+  // rather than in the file because it must not go insensitive with the body: turning trigger mode
+  // back on would otherwise mean finding the toolbar button that greyed the panel out.
+  GtkWidget* head = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 8);
+  gtk_widget_set_margin_top(head, 8);
+  gtk_widget_set_margin_start(head, 8);
+  gtk_widget_set_margin_end(head, 8);
+  GtkWidget* title = gtk_label_new("Trigger");
+  gtk_widget_set_halign(title, GTK_ALIGN_START);
+  gtk_widget_set_hexpand(title, TRUE);
+  gtk_box_append(GTK_BOX(head), title);
+  app.trigger_switch = gtk_switch_new();
+  gtk_widget_set_valign(app.trigger_switch, GTK_ALIGN_CENTER);
+  gtk_widget_set_tooltip_text(app.trigger_switch,
+                              "trigger mode, the same switch the header bar carries");
+  g_signal_connect(app.trigger_switch, "notify::active", G_CALLBACK(on_panel_switch_toggled), &app);
+  gtk_box_append(GTK_BOX(head), app.trigger_switch);
+
+  GtkWidget* outer = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
+  gtk_box_append(GTK_BOX(outer), head);
+  gtk_box_append(GTK_BOX(outer), box);
+  return outer;
 }
 
 }  // namespace ui_impl