Change grid to box

data/ui/countdown.ui

@@ -25,9 +25,15 @@
   <template class="ApplicationWindow" parent="GtkApplicationWindow">
     <property name="can_focus">False</property>
     <child>
-      <object class="GtkGrid" id="_main_grid">
+      <object class="GtkBox" id="_main_box">
         <property name="can_focus">False</property>
         <property name="orientation">vertical</property>
+        <child>
+          <placeholder/>
+        </child>
+        <child>
+          <placeholder/>
+        </child>
       </object>
     </child>
     <child type="titlebar">

src/applicationwindow.py

@@ -12,7 +12,7 @@ from .timepopover import TimePopover
 class ApplicationWindow(Gtk.ApplicationWindow):
     __gtype_name__ = 'ApplicationWindow'
 
-    _main_grid = Gtk.Template.Child()
+    _main_box = Gtk.Template.Child()
     _set_time_button = Gtk.Template.Child()
 
     def __init__(self, app):
@@ -25,9 +25,9 @@ class ApplicationWindow(Gtk.ApplicationWindow):
         )
 
         self._clock = DigitalClock()
-        self._main_grid.add(self._clock)
+        self._main_box.add(self._clock)
         self._count = CountdownLabel()
-        self._main_grid.add(self._count)
+        self._main_box.add(self._count)
 
     def _on_set_time_popover_notify_datetime(self, popover, pspec):
         self._count.target = popover.props.datetime