login: use default widget for logging on enter press

Expose the login button in login.ui to retrieve it by the default_widget function in login.
Set the login  button as the default widget when the visible child in main_stack is login,
and connect the function to the visible_child change signal to update the default widget.

fix: https://gitlab.gnome.org/GNOME/fractal/-/issues/788

data/resources/ui/login.ui

@@ -12,7 +12,7 @@
               </object>
             </property>
             <child type="end">
-              <object class="GtkButton">
+              <object class="GtkButton" id="next_button">
                 <property name="action_name">login.next</property>
                 <property name="child">
                   <object class="GtkStack" id="next_stack">

src/login.rs

@@ -17,6 +17,8 @@ mod imp {
     #[derive(Debug, Default, CompositeTemplate)]
     #[template(resource = "/org/gnome/FractalNext/login.ui")]
     pub struct Login {
+        #[template_child]
+        pub next_button: TemplateChild<gtk::Button>,
         #[template_child]
         pub next_stack: TemplateChild<gtk::Stack>,
         #[template_child]
@@ -186,6 +188,10 @@ impl Login {
         })
         .unwrap()
     }
+
+    pub fn default_widget(&self) -> gtk::Widget {
+        imp::Login::from_instance(&self).next_button.get().upcast()
+    }
 }
 
 fn build_homeserver_url(server: &str) -> Result<Url, ParseError> {

src/window.rs

@@ -65,6 +65,11 @@ mod imp {
             self.login.connect_new_session(
                 clone!(@weak obj => move |_login, session| obj.add_session(session)),
             );
+
+            self.main_stack.connect_visible_child_notify(
+                clone!(@weak obj => move |_| obj.set_default_by_child()),
+            );
+            obj.set_default_by_child();
         }
     }
 
@@ -148,4 +153,15 @@ impl Window {
         self.set_default_size(width, height);
         self.set_property("maximized", &is_maximized).unwrap();
     }
+
+    /// Change the default widget of the window based on the visible child
+    /// If the login screen is visible, its login button becomes the default widget
+    fn set_default_by_child(&self) {
+        let priv_ = imp::Window::from_instance(self);
+        if priv_.main_stack.visible_child() == Some(priv_.login.get().upcast()) {
+            self.set_default_widget(Some(&priv_.login.default_widget()));
+        } else {
+            self.set_default_widget(gtk::NONE_WIDGET);
+        }
+    }
 }