views/month-cell: Override focus() virtual function

Without this patch, focus immediately goes to the "Previous Month"
button at startup, for some unknown reason.
So, as a workaround, skip focus if the root cannot retrieve the
current focused widget.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/564>

src/gui/views/gcal-month-cell.c

@@ -344,6 +344,23 @@ on_breakpoint_changed_cb (GcalMonthCell *self)
 }
 
 
+/*
+ * GtkWidget overrides
+ */
+
+static gboolean
+gcal_month_cell_focus (GtkWidget        *widget,
+                       GtkDirectionType  direction)
+{
+  GtkRoot *root = gtk_widget_get_root (widget);
+
+  if (gtk_root_get_focus (root) == NULL)
+    return FALSE;
+
+  return GTK_WIDGET_CLASS (gcal_month_cell_parent_class)->focus (widget, direction);
+}
+
+
 /*
  * GObject overrides
  */
@@ -414,6 +431,8 @@ gcal_month_cell_class_init (GcalMonthCellClass *klass)
   object_class->set_property = gcal_month_cell_set_property;
   object_class->get_property = gcal_month_cell_get_property;
 
+  widget_class->focus = gcal_month_cell_focus;
+
   signals[SHOW_OVERFLOW] = g_signal_new ("show-overflow",
                                          GCAL_TYPE_MONTH_CELL,
                                          G_SIGNAL_RUN_LAST,