ui: a press anywhere in the window closes the time-window popover

The window control is a GtkMenuButton whose popover autohides -- which works for as long as
GTK holds the popover's grab, and the grab is what can go missing: a window manager moving
it, the nested popover the unit dropdown opens taking it, and every headless run, where
there is no manager to hand one out at all. After that a click anywhere else in the window
leaves the bubble up, which is the "sometimes it is awkward to close" report.

`on_window_popover_press()` states the rule outright instead of relying on the grab: any
press that reaches the window's own surface closes it. That needs no hit test beyond the
button -- a popover is its own surface, so a click *inside* it never arrives here -- and the
button is left alone, because its own click is the toggle that closes it. The press gesture
is on the window in the capture phase, so a widget under the press (the plot's own gestures
among them) cannot hide it.

[windowpop] reads both halves, with autohide turned off for the probe -- the state a user
meets when the grab goes missing:

    [windowpop] opened=1 visible=1 click_gestures=2
    [windowpop] after a press elsewhere: active=0 (want 0)
    [windowpop] after a press on the button: active=1 (want 1)

`click_gestures` is read as well as the handler, so "the rule works" and "the rule is
connected" are two facts rather than one.

src/ui_probe.cpp

@@ -2500,6 +2500,63 @@ void menus_probe_next(App& a, int step, guint delay_ms) {
   g_timeout_add(delay_ms, menus_probe_step, &a);
 }
 
+/// `UDPSCOPE_PROBE_WINDOWPOP_MS`: the time-window popover, closed by a press elsewhere in the window.
+///
+/// Autohide is turned off for the probe, exactly as the menu probe does it: a popover dismisses itself
+/// when GTK holds its *grab*, and a bare `xvfb-run` has no window manager to hand one out -- so with
+/// autohide on the bubble would not stay up to be tested. Autohide off is the state a user meets when
+/// the grab goes missing, which is the report this guards: the press has to close it anyway. The second
+/// half presses the button itself, which must *not* close it (its own click toggles).
+gboolean windowpop_probe_step(gpointer user) {
+  static int step = 0;
+  App* a = static_cast<App*>(user);
+  if (a->window == nullptr || a->window_button == nullptr) return G_SOURCE_REMOVE;
+  const auto active = [a] {
+    return gtk_menu_button_get_active(GTK_MENU_BUTTON(a->window_button)) ? 1 : 0;
+  };
+  if (step == 0) {
+    GtkPopover* pop = gtk_menu_button_get_popover(GTK_MENU_BUTTON(a->window_button));
+    if (pop != nullptr) gtk_popover_set_autohide(pop, FALSE);
+    gtk_menu_button_popup(GTK_MENU_BUTTON(a->window_button));
+    step = 1;
+    g_timeout_add(300, windowpop_probe_step, a);
+    return G_SOURCE_REMOVE;
+  }
+  // How many click gestures the window carries, so that "the handler works" and "the handler is
+  // connected" are read as two facts rather than one.
+  int clicks = 0;
+  {
+    GListModel* controllers = gtk_widget_observe_controllers(a->window);
+    const guint n = controllers != nullptr ? g_list_model_get_n_items(controllers) : 0;
+    for (guint i = 0; i < n; ++i) {
+      GObject* item = G_OBJECT(g_list_model_get_item(controllers, i));
+      if (item != nullptr) {
+        if (GTK_IS_GESTURE_CLICK(item)) ++clicks;
+        g_object_unref(item);
+      }
+    }
+    if (controllers != nullptr) g_object_unref(controllers);
+  }
+  std::fprintf(stderr, "[windowpop] opened=%d visible=%d click_gestures=%d\n", active(),
+               gtk_widget_is_visible(GTK_WIDGET(gtk_menu_button_get_popover(
+                   GTK_MENU_BUTTON(a->window_button))))
+                   ? 1
+                   : 0,
+               clicks);
+  on_window_popover_press(nullptr, 0, 20.0, 200.0, a);
+  std::fprintf(stderr, "[windowpop] after a press elsewhere: active=%d (want 0)\n", active());
+  gtk_menu_button_popup(GTK_MENU_BUTTON(a->window_button));
+  graphene_rect_t r = GRAPHENE_RECT_INIT(0, 0, 0, 0);
+  if (gtk_widget_compute_bounds(a->window_button, a->window, &r)) {
+    on_window_popover_press(nullptr, 0, r.origin.x + r.size.width / 2.0,
+                            r.origin.y + r.size.height / 2.0, a);
+  }
+  std::fprintf(stderr, "[windowpop] after a press on the button: active=%d (want 1)\n", active());
+  gtk_menu_button_popdown(GTK_MENU_BUTTON(a->window_button));
+  std::fflush(stderr);
+  return G_SOURCE_REMOVE;
+}
+
 gboolean menus_probe_step(gpointer user) {
   App* a = static_cast<App*>(user);
   if (a->window == nullptr) return G_SOURCE_REMOVE;