fix: the overlaid sidebar steps aside for a drag, and the compact view follows the trigger

Two reports, both real, and neither was about the lock itself.

**A drag out of an overlaid sidebar had nowhere to land.** In a window too narrow to show both columns,
`AdwOverlaySplitView` puts the sidebar *on* the plot — that is what `collapsed` means — so the pointer
stays over the list and never over a drop target: a signal could be dragged but not dropped, which reads
as "dragging does not work". `row_drag_sidebar()` now hides the sidebar on `drag-begin` and restores it on
`drag-end`, and only when collapsed: side by side the plot is already visible, and hiding the list the drag
started in would be startling. The reading begins a drag through the *source* with the state forced
collapsed, the way the sidebar probe forces it:

    pane drag: sidebar hidden for the drag=1 restored after it=1

**The compact view did not follow the trigger switch.** The locked line's first branch reads
`tc.enabled` — with trigger mode off it says `stream (trigger off)` and nothing else — and
`on_trigger_toggled()` never re-synced it, so the line kept saying whichever it had been. One call to
`sync_trigger_summary()` beside the switch sync. The dump's two `panes:` lines were already the test and
now pass it: with a session whose trigger is off, the line as found reads `stream (trigger off)` and the
live one reads the settings, because the exit probe turns the trigger on in between. Both reading `stream`
was the defect.

Verified: the two lines above; `panes: the same line as the probe found it: 'mode: stream (trigger off)'`
against the live `'signal: square | condition: window, rising | level: 0.3 .. 0.6 | mode: normal'`;
`make test-ui` 14 files; zero `Gtk-CRITICAL`. Docs: the `pane drag:` reading and the invariants bullet for
the overlay, and the `panes:` pair's second job — the refresh test — written down so the next reader knows
what both lines reading `stream` means.

src/ui.cpp

@@ -4436,6 +4436,25 @@ void on_pane_edit_clicked(GtkButton* button, gpointer user) {
   apply_pane_lock(*app);
 }
 
+/// Hides the sidebar for the length of a drag, when it is *overlaid*, and restores it after.
+///
+/// With the split view collapsed the sidebar covers the plot -- that is what collapsed means -- so a drag
+/// that starts in the signal list has nowhere to land: the pointer is over the sidebar and never over a
+/// drop target, and the drag is impossible rather than merely awkward. Side by side nothing is done: the
+/// plot is already visible, and hiding the list the drag started in would be startling.
+void row_drag_sidebar(App& app, bool dragging) {
+  if (app.side_view == nullptr) return;
+  AdwOverlaySplitView* split = ADW_OVERLAY_SPLIT_VIEW(app.side_view);
+  if (dragging) {
+    if (!adw_overlay_split_view_get_collapsed(split)) return;
+    app.sidebar_hidden_for_drag = adw_overlay_split_view_get_show_sidebar(split) != FALSE;
+    if (app.sidebar_hidden_for_drag) adw_overlay_split_view_set_show_sidebar(split, FALSE);
+  } else if (app.sidebar_hidden_for_drag) {
+    app.sidebar_hidden_for_drag = false;
+    adw_overlay_split_view_set_show_sidebar(split, TRUE);
+  }
+}
+
 void run_app_action(App& app, AppAction action) {
   const auto toggle = [](GtkWidget* button) {
     if (button == nullptr) return;
@@ -5782,7 +5801,9 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 app.window_limit_s);
     {
       GtkIconTheme* theme = gtk_icon_theme_get_for_display(gdk_display_get_default());
-      std::printf("[udpscope]   pane edits: locked=%s(%s) config=%s(%s)\n",
+      std::printf("[udpscope]   pane drag: sidebar hidden for the drag=%d restored after it=%d\n",
+                app.dbg_drag_hid_sidebar, app.dbg_drag_restored_sidebar);
+    std::printf("[udpscope]   pane edits: locked=%s(%s) config=%s(%s)\n",
                   app.dbg_pane_edit_icon.c_str(),
                   gtk_icon_theme_has_icon(theme, app.dbg_pane_edit_icon.c_str()) ? "ok" : "MISSING",
                   app.dbg_pane_view_icon.c_str(),

src/ui_app.hpp

@@ -261,6 +261,9 @@ struct App {
   /// 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;
+  /// True while a drag has the overlaid sidebar hidden: what it was showing before, so `drag-end` puts it
+  /// back rather than guessing.
+  bool sidebar_hidden_for_drag = false;
   /// The channel a session's trigger names, until `rebuild_trigger_signals()` can resolve it: the
   /// file stores a *name* and the trigger needs an index into the acquired channels, which do not
   /// exist until the stream has been described. Empty means "nothing pending".
@@ -822,6 +825,9 @@ struct App {
   int dbg_signal_rows = 0;
   /// How many rows of the list carry a `GtkDragSource`: `rows - 1`, the divider not being draggable.
   int dbg_signal_drag_sources = -1;
+  /// The drag-with-an-overlaid-sidebar pair: the sidebar hid for the drag, and came back after it.
+  int dbg_drag_hid_sidebar = -1;
+  int dbg_drag_restored_sidebar = -1;
   std::size_t dbg_signal_plain = 0;
   int dbg_side_column_after_hide = 1;
   std::string dbg_mode_triggered;
@@ -1462,6 +1468,10 @@ const char* trigger_mode_text(TriggerMode m);
 TriggerMode trigger_mode_from_text(const std::string& s);
 /// The panel's controls from the model, and the session's trigger into both.
 void write_trigger_widgets(App& app);
+/// Hides the sidebar for the length of a drag when it is *overlaid*, and puts it back after. With the
+/// split view collapsed the sidebar covers the plot, so a drag that starts in the signal list has nowhere
+/// to land: the pointer is over the sidebar, not over a drop target.
+void row_drag_sidebar(App& app, bool dragging);
 /// The compact line the trigger pane shows while it is locked, and the lock itself: the panes'
 /// bodies, their [edit] buttons and the signal list's rows all follow from `trigger_locked` and
 /// `signals_locked`.

src/ui_probe.cpp

@@ -103,6 +103,35 @@ gboolean exit_probe_step3(gpointer user) {
     }
     a->dbg_signal_drag_sources = sources;
   }
+  // A drag out of an *overlaid* sidebar: the sidebar has to step aside or the drop has nowhere to land.
+  // The state is forced the way the sidebar probe forces it -- collapsed is what a window with no room
+  // beside the plot produces -- and the drag is begun through the *source*, so the handler is the one a
+  // real drag reaches.
+  if (a->side_view != nullptr && a->signal_list != nullptr) {
+    AdwOverlaySplitView* split = ADW_OVERLAY_SPLIT_VIEW(a->side_view);
+    GtkWidget* row = gtk_widget_get_first_child(a->signal_list);
+    adw_overlay_split_view_set_collapsed(split, TRUE);
+    adw_overlay_split_view_set_show_sidebar(split, TRUE);
+    if (row != nullptr) {
+      GListModel* controllers = gtk_widget_observe_controllers(row);
+      const guint n = controllers != nullptr ? g_list_model_get_n_items(controllers) : 0;
+      for (guint i = 0; i < n; ++i) {
+        GObject* c = G_OBJECT(g_list_model_get_item(controllers, i));
+        if (GTK_IS_DRAG_SOURCE(c)) {
+          g_signal_emit_by_name(c, "drag-begin", nullptr);
+          a->dbg_drag_hid_sidebar =
+              adw_overlay_split_view_get_show_sidebar(split) == FALSE ? 1 : 0;
+          row_drag_sidebar(*a, false);
+          a->dbg_drag_restored_sidebar =
+              adw_overlay_split_view_get_show_sidebar(split) != FALSE ? 1 : 0;
+        }
+        g_object_unref(c);
+      }
+    }
+    // Left as the window's own width decides, so the readings taken before this are the ones that stand.
+    adw_overlay_split_view_set_collapsed(split, FALSE);
+    adw_overlay_split_view_set_show_sidebar(split, TRUE);
+  }
   // 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

src/ui_signals.cpp

@@ -457,6 +457,17 @@ void rebuild_signal_list(App& app, const std::vector<SignalInfo>& signals) {
     g_signal_connect_data(source, "prepare", G_CALLBACK(on_drag_prepare), ctx,
                           [](gpointer data, GClosure*) { delete static_cast<DragContext*>(data); },
                           G_CONNECT_DEFAULT);
+    // A drag out of an *overlaid* sidebar would have nowhere to land (see `row_drag_sidebar()`), so the
+    // sidebar steps out of the way for as long as the drag lasts.
+    g_signal_connect(source, "drag-begin", G_CALLBACK(+[](GtkDragSource*, GdkDrag*, gpointer user) {
+                       row_drag_sidebar(*static_cast<App*>(user), true);
+                     }),
+                     &app);
+    g_signal_connect(source, "drag-end",
+                     G_CALLBACK(+[](GtkDragSource*, GdkDrag*, gboolean, gpointer user) {
+                       row_drag_sidebar(*static_cast<App*>(user), false);
+                     }),
+                     &app);
     gtk_widget_add_controller(row, GTK_EVENT_CONTROLLER(source));
 
     gtk_list_box_append(GTK_LIST_BOX(list), row);

src/ui_trigger.cpp

@@ -413,8 +413,10 @@ void on_trigger_toggled(GtkToggleButton* button, gpointer user) {
   // Arming is a *runtime* control: with trigger mode off there is no engine to arm, and the button is the
   // one part of this pane that is not a setting. (The body itself stays live -- see above.)
   if (app->arm_button != nullptr) gtk_widget_set_sensitive(app->arm_button, on);
-  // The pane's own switch shows the same state, whichever of the two was clicked.
+  // The pane's own switch shows the same state, whichever of the two was clicked, and the compact line
+  // changes shape with it: with trigger mode off it says "stream" and nothing else.
   sync_panel_switches(*app);
+  sync_trigger_summary(*app);
 }
 
 /// The way out of trigger mode, from inside the panel that froze the view: it is