gui/event-widget: Guard event popover's pointing coordinates on multi-day events

A follow-up on https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/677#note_2628484,
which prevents the event popover from pointing outside the event
widget's boundaries on multi-day events when activated before exceeding
the drag threshold:
https://docs.gtk.org/gtk4/property.Settings.gtk-dnd-drag-threshold.html

This is mainly useful for people who fidget and may sloppily
click and drag event widgets (before triggering the drag operation),
causing the popover to place itself in unpredictable locations.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/697>

src/gui/gcal-event-widget.c

@@ -514,8 +514,9 @@ on_click_gesture_release_cb (GtkGestureClick *click_gesture,
                              gdouble          y,
                              GcalEventWidget *self)
 {
-  GdkRectangle rect;
   gboolean not_inside_list_box;
+  GdkRectangle rect;
+  gint half_width;
 
   gtk_gesture_set_state (GTK_GESTURE (click_gesture), GTK_EVENT_SEQUENCE_CLAIMED);
   g_signal_emit (self, signals[ACTIVATE], 0);
@@ -525,10 +526,13 @@ on_click_gesture_release_cb (GtkGestureClick *click_gesture,
   not_inside_list_box = !gtk_widget_get_ancestor (GTK_WIDGET (self), GTK_TYPE_LIST_BOX);
   if (not_inside_list_box && gcal_event_is_multiday (self->event))
     {
-      rect.x = x;
+      rect.width = gtk_widget_get_width (GTK_WIDGET (self));
+      rect.height = gtk_widget_get_height (GTK_WIDGET (self));
+
+      half_width = rect.width / 2;
+
+      rect.x = CLAMP (x - half_width, -half_width, half_width);
       rect.y = 0;
-      rect.width = 1;
-      rect.height = 1;
 
       gtk_popover_set_pointing_to (GTK_POPOVER (self->preview_popover), &rect);
     }