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.cpp

@@ -4114,6 +4114,32 @@ GtkWidget* build_window_control(App& app) {
   return app.window_button;
 }
 
+/// Any press in the window closes the time-window popover.
+///
+/// `GtkPopover`'s autohide does this by itself for as long as GTK holds the popover's grab -- and the grab
+/// is the thing that can go missing (a window manager moving it, a nested popover taking it for the unit
+/// menu, and every headless run, where there is no manager to hand one out at all), after which a click
+/// anywhere else in the window leaves the bubble up. That is the "sometimes it is awkward to close".
+///
+/// A press that reaches the window's own surface was *not* inside the popover: a popover is its own
+/// surface, so a click within it never arrives here. That is what makes this the rule stated outright
+/// rather than a grab away from working -- and it needs no hit test beyond leaving the button alone,
+/// whose own click is what toggles the bubble shut.
+void on_window_popover_press(GtkGestureClick*, int, double x, double y, gpointer user) {
+  App* app = static_cast<App*>(user);
+  if (app->window_button == nullptr ||
+      !gtk_menu_button_get_active(GTK_MENU_BUTTON(app->window_button))) {
+    return;
+  }
+  // The button itself is left to its own click, which toggles the bubble shut -- closing it here would
+  // only have the toggle open it again. Anything else on the window's surface closes it.
+  for (GtkWidget* w = gtk_widget_pick(app->window, x, y, GTK_PICK_DEFAULT);
+       w != nullptr && w != app->window; w = gtk_widget_get_parent(w)) {
+    if (w == app->window_button) return;
+  }
+  gtk_menu_button_popdown(GTK_MENU_BUTTON(app->window_button));
+}
+
 /// The one place a key turns into an action. Toggles go through
 /// 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
@@ -4659,6 +4685,13 @@ void on_activate(GtkApplication* application, gpointer user) {
   g_signal_connect(keys, "key-pressed", G_CALLBACK(on_app_key_pressed), app);
   gtk_widget_add_controller(app->window, keys);
 
+  // Capture phase, unlike the keys above: a press belongs to whatever widget is under it (the plot's
+  // gestures among them), and the point here is to see every one of them, not to win none of them.
+  GtkGesture* press = gtk_gesture_click_new();
+  gtk_event_controller_set_propagation_phase(GTK_EVENT_CONTROLLER(press), GTK_PHASE_CAPTURE);
+  g_signal_connect(press, "pressed", G_CALLBACK(on_window_popover_press), app);
+  gtk_widget_add_controller(app->window, GTK_EVENT_CONTROLLER(press));
+
   gtk_window_present(GTK_WINDOW(app->window));
 }
 
@@ -5279,6 +5312,12 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   if (const char* env = g_getenv("UDPSCOPE_PROBE_KEYS_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), keys_probe, &app);
   }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_WINDOWPOP_MS")) {
+    g_timeout_add(static_cast<guint>(std::atoi(env)), windowpop_probe_step, &app);
+  }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_KEYS_MS")) {
+    g_timeout_add(static_cast<guint>(std::atoi(env)), keys_probe, &app);
+  }
   if (const char* env = g_getenv("UDPSCOPE_PROBE_COLOR_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), color_probe, &app);
   }