feat: the cursors panel is a UI file

src/ui/cursors-panel.ui holds the panel's shape -- the header with the mode dropdown and the reset
button, and the grid the two cursor times are read in. What is not in it is the grid's contents,
which follow the cursor mode and the rows and are rebuilt whenever either changes.

Panels need a builder that outlives them, unlike the dialogs, which are rebuilt on every opening: a
widget belongs to the builder that built it, so App::ui_builders holds each panel's for the life of
the window. ui_load() is that, in ui_widgets.cpp.

make test-ui also checks that udpscope-ui.gresource.xml lists every .ui file now. That was the third
trap of this refactor -- a file on disk but not in the resource is a Gtk-ERROR at load, which aborts
-- and it is a compile-time mistake that nothing else in the build could see. Verified by unlisting
the new file and watching the check fail, then restoring it.

[cursors-probe] reads as it did: all four modes, pane=1, the timings and the drag cases.

src/ui/cursors-panel.ui

@@ -0,0 +1,50 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!--
+  The cursors panel: a header with the mode's dropdown and the reset button, and the grid the two
+  cursor times are read in, one line per row of the figure.
+
+  The grid's *contents* are not here: they follow the cursor mode and the rows, and they are rebuilt
+  whenever either changes (`rebuild_cursors_pane()`). What is here is the panel's shape, the dropdown
+  and the button, and the grid they sit above.
+-->
+<interface>
+  <object class="GtkBox" id="cursors_panel">
+    <property name="orientation">vertical</property>
+    <property name="spacing">4</property>
+    <property name="margin-top">6</property>
+    <property name="margin-bottom">6</property>
+    <property name="margin-start">6</property>
+    <property name="margin-end">6</property>
+    <child>
+      <object class="GtkBox" id="head">
+        <property name="orientation">horizontal</property>
+        <property name="spacing">8</property>
+        <child>
+          <object class="GtkLabel" id="title">
+            <property name="label" translatable="yes">Cursors</property>
+            <property name="halign">start</property>
+            <property name="hexpand">1</property>
+          </object>
+        </child>
+        <child>
+          <object class="GtkDropDown" id="mode_drop"/>
+        </child>
+        <child>
+          <object class="GtkButton" id="reset">
+            <property name="icon-name">edit-clear-symbolic</property>
+            <property name="valign">center</property>
+            <property name="css-classes">flat</property>
+          </object>
+        </child>
+      </object>
+    </child>
+    <child>
+      <object class="GtkGrid" id="body">
+        <property name="row-spacing">2</property>
+        <property name="column-spacing">4</property>
+      </object>
+    </child>
+  </object>
+</interface>

src/ui/udpscope-ui.gresource.xml

@@ -14,5 +14,6 @@
     <file preprocess="xml-stripblanks">help.ui</file>
     <file preprocess="xml-stripblanks">source-form.ui</file>
     <file preprocess="xml-stripblanks">main-window.ui</file>
+    <file preprocess="xml-stripblanks">cursors-panel.ui</file>
   </gresource>
 </gresources>

src/ui_app.hpp

@@ -399,6 +399,10 @@ struct App {
   plot::Figure* area_b_figure = nullptr;
   std::unique_ptr<RowSet> rows_b;
   GtkWidget* plot_split = nullptr;
+  /// The builders of the panels inside the window. A widget belongs to the builder that built it, and a
+  /// panel lives as long as the window, so these do too -- unlike the dialogs', which are replaced.
+  std::vector<GtkBuilder*> ui_builders;
+
   /// The main window's builder. Held because the window belongs to its object list: it is replaced on
   /// the next build rather than reused, and everything found by id comes out of it.
   GtkBuilder* window_ui = nullptr;
@@ -1038,6 +1042,9 @@ GtkWidget* menu_button(const char* label, const char* accel);
 GtkWidget* menu_toggle(const char* label, const char* accel, bool active);
 GtkWidget* menu_separator();
 GtkWidget* labeled_box(const char* label, GtkWidget* child);
+/// Loads a UI file into the window's builders and returns the builder, so the caller can resolve the
+/// objects it needs by id. See `ui_widgets.cpp`.
+GtkBuilder* ui_load(App& app, const char* resource);
 LayoutState collect_layout(App& app);
 void on_config_row_activated(GtkListBox*, GtkListBoxRow* row, gpointer user);
 void on_config_forget_clicked(GtkButton* button, gpointer user);

src/ui_cursors.cpp

@@ -78,41 +78,38 @@ void update_cursors_pane(App& app) {
 }
 
 GtkWidget* build_cursors_panel(App& app) {
-  GtkWidget* box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 4);
-  gtk_widget_set_margin_top(box, 6);
-  gtk_widget_set_margin_bottom(box, 6);
-  gtk_widget_set_margin_start(box, 6);
-  gtk_widget_set_margin_end(box, 6);
+  // The panel's shape is a UI file; the *contents* of its grid are not, because they follow the cursor
+  // mode and the rows and are rebuilt whenever either changes.
+  GtkBuilder* ui = ui_load(app, "/org/udpscope/ui/cursors-panel.ui");
+  GtkWidget* box = GTK_WIDGET(gtk_builder_get_object(ui, "cursors_panel"));
+  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"));
+  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
+    // with it, and `[cursors]` reads differently rather than the application failing to start.
+    return box != nullptr ? box : gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
+  }
 
-  GtkWidget* head = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 8);
-  GtkWidget* title = gtk_label_new("Cursors");
-  gtk_widget_set_halign(title, GTK_ALIGN_START);
-  gtk_widget_set_hexpand(title, true);
-  gtk_box_append(GTK_BOX(head), title);
+  // The mode names are the dropdown's model, and they are a C array, so they are set here.
   const char* names[5] = {kCursorModeNames[0], kCursorModeNames[1], kCursorModeNames[2],
                           kCursorModeNames[3], nullptr};
-  app.cursors_mode_drop = gtk_drop_down_new_from_strings(names);
+  gtk_drop_down_set_model(GTK_DROP_DOWN(app.cursors_mode_drop),
+                          G_LIST_MODEL(gtk_string_list_new(names)));
   gtk_widget_set_tooltip_text(app.cursors_mode_drop,
                               "vertical: two times, shared by every row\n"
-                              "horizontal: two rulers per row\n"
-                              "both: the two together\n"
+                              "horizontal: one value per row, at the two times\n"
+                              "both: the two times and one value per row\n"
                               "mix: the two times, read off each trace where they cross");
   g_signal_connect(app.cursors_mode_drop, "notify::selected",
                    G_CALLBACK(on_cursor_mode_changed), &app);
-  gtk_box_append(GTK_BOX(head), app.cursors_mode_drop);
-  // Reset lives here, beside the mode it resets, rather than in the overflow menu: it acts on this
-  // pane's instruments and is only visible while they are on, both of which are this pane's facts.
-  app.cursors_reset = gtk_button_new_from_icon_name("edit-clear-symbolic");
-  gtk_widget_add_css_class(app.cursors_reset, "flat");
+
+  // Reset lives beside the mode it resets rather than in the overflow menu: it acts on this pane's
+  // instruments and is only visible while they are on, both of which are this pane's facts.
   gtk_widget_set_tooltip_text(app.cursors_reset, "re-place the cursors at the window's edges");
   g_signal_connect(app.cursors_reset, "clicked", G_CALLBACK(on_cursors_reset), &app);
-  gtk_box_append(GTK_BOX(head), app.cursors_reset);
-  gtk_box_append(GTK_BOX(box), head);
 
-  app.cursors_body = gtk_grid_new();
-  gtk_grid_set_row_spacing(GTK_GRID(app.cursors_body), 2);
-  gtk_grid_set_column_spacing(GTK_GRID(app.cursors_body), 4);
-  gtk_box_append(GTK_BOX(box), app.cursors_body);
   return box;
 }
 

src/ui_widgets.cpp

@@ -12,6 +12,22 @@
 namespace udpscope {
 namespace ui_impl {
 
+/// Loads one UI file and keeps its builder for the life of the window.
+///
+/// A widget belongs to the builder that built it -- drop the builder and the widgets it made go with
+/// it -- so the panels' builders are kept in `App` beside the widgets they built. Unlike the dialogs
+/// (`prefs_open()`, `show_help()`), a panel is built once and lives as long as the window, so these are
+/// appended and never replaced.
+///
+/// A resource that is not in the GResource aborts here rather than returning null: `GtkBuilder` reports
+/// a missing resource with a `Gtk-ERROR`, which is fatal. `make test-ui` checks that the xml lists every
+/// `.ui` file, which is the only way to make that a build-time failure instead.
+GtkBuilder* ui_load(App& app, const char* resource) {
+  GtkBuilder* builder = gtk_builder_new_from_resource(resource);
+  app.ui_builders.push_back(builder);
+  return builder;
+}
+
 /// Installs the one style rule the app needs: a signal row that is as short as its
 /// contents.
 ///