pantheon.elementary-greeter: actually fix the crash

Fixed a use-after-free issue where logged_in_context is used in update_style().
There are several reports for this but upstream has no action for this so far during the 6.x cycle.
See the provided link for more details.

(cherry picked from commit f29955d)

Note: the above is the original commit message, the patch has been accepted on upstream.

pkgs/desktops/pantheon/desktop/elementary-greeter/default.nix

@@ -94,6 +94,9 @@ stdenv.mkDerivation rec {
       src = ./hardcode-fallback-background.patch;
       default_wallpaper = "${nixos-artwork.wallpapers.simple-dark-gray.gnomeFilePath}";
     })
+    # https://github.com/NixOS/nixpkgs/issues/151609
+    # https://github.com/elementary/greeter/issues/578#issuecomment-1030746697
+    ./fix-crash.patch
   ];
 
   preFixup = ''

pkgs/desktops/pantheon/desktop/elementary-greeter/fix-crash.patch

@@ -0,0 +1,21 @@
+diff --git a/src/Cards/UserCard.vala b/src/Cards/UserCard.vala
+index 83df22c..8cd917e 100644
+--- a/src/Cards/UserCard.vala
++++ b/src/Cards/UserCard.vala
+@@ -42,6 +42,7 @@ public class Greeter.UserCard : Greeter.BaseCard {
+     private Gtk.Stack login_stack;
+     private Greeter.PasswordEntry password_entry;
+
++    private SelectionCheck logged_in;
+     private unowned Gtk.StyleContext logged_in_context;
+     private weak Gtk.StyleContext main_grid_style_context;
+     private weak Gtk.StyleContext password_entry_context;
+@@ -214,7 +215,7 @@ public class Greeter.UserCard : Greeter.BaseCard {
+         };
+         avatar_overlay.add (avatar);
+
+-        var logged_in = new SelectionCheck () {
++        logged_in = new SelectionCheck () {
+             halign = Gtk.Align.END,
+             valign = Gtk.Align.END
+         };