fix(overlay): patch font-manager 0.9.4 for gtk4 4.22 Vala errors

font-manager 0.9.4 fails to compile against gtk4 4.22: the
Gtk.DragIcon.get_for_drag binding changed in the gtk4 vapi, producing
'use new operator' errors in Collections.vala and FontList.vala that
broke home-manager-path for all desktop hosts.

0.9.4 is the latest upstream release (master unfixed too); the fix is
upstream PR FontManager/font-manager#468, already carried by nixpkgs
master but predating our locked nixpkgs and nixpkgs-master inputs.
Vendor the patch as an overlay mod, following the aardvark-dns pattern.
Drop once the nixpkgs lock carries it.

overlays/default.nix

@@ -7,10 +7,15 @@
   # This one contains whatever you want to overlay
   # You can change versions, add patches, set compilation flags, anything really.
   # https://nixos.wiki/wiki/Overlays
-  modifications = final: _prev: {
+  modifications = final: prev: {
     # aardvark-dns 2.x has a DNS forwarding bug breaking external name
     # resolution in podman networks — pin to last 1.x from nixpkgs-stable.
     aardvark-dns = import ./mods/aardvark-dns.nix {inherit (final) stable;};
+
+    # font-manager 0.9.4 does not compile against gtk4 4.22 (Vala errors
+    # in Gtk.DragIcon.get_for_drag) — apply the upstream fix from
+    # FontManager/font-manager#468 until our nixpkgs lock carries it.
+    font-manager = import ./mods/font-manager.nix {inherit prev;};
   };
 
   # temp-packages = final: _prev: {

overlays/mods/font-manager.nix

@@ -0,0 +1,19 @@
+# font-manager 0.9.4: fix compilation against gtk4 >= 4.22 / newer Vala.
+#
+# In GTK 4.22 the Gtk.DragIcon.get_for_drag binding changed in the gtk4
+# vapi, turning `Gtk.DragIcon.get_for_drag(drag)` into a hard Vala error
+# ("use `new' operator to create new objects") in Collections.vala and
+# FontList.vala.  0.9.4 is the latest upstream release and even upstream
+# master is unfixed; the fix lives in FontManager/font-manager#468 and is
+# already carried as a patch by current nixpkgs master — but our locked
+# nixpkgs (and the locked nixpkgs-master input) predate it.
+#
+# Drop this overlay (and the patch file) once our nixpkgs lock advances
+# past nixpkgs@e2ad529-vendoring (pkgs/by-name/fo/font-manager carries
+# fix-compilation-error-with-newer-vala-versions.patch).
+{prev}:
+prev.font-manager.overrideAttrs (old: {
+  patches =
+    (old.patches or [])
+    ++ [./patches/font-manager-fix-compilation-error-with-newer-vala-versions.patch];
+})

overlays/mods/patches/font-manager-fix-compilation-error-with-newer-vala-versions.patch

@@ -0,0 +1,37 @@
+From e2ad529a88929bbc76906ac78260dacf4d8c8c6b Mon Sep 17 00:00:00 2001
+From: Jan Baier <jan.baier@amagical.net>
+Date: Fri, 1 May 2026 17:25:25 +0200
+Subject: [PATCH] Fix compilation error with newer Vala versions
+
+Fixes #467
+---
+ src/font-manager/Collections.vala | 2 +-
+ src/font-manager/FontList.vala    | 2 +-
+ 2 files changed, 2 insertions(+), 2 deletions(-)
+
+diff --git a/src/font-manager/Collections.vala b/src/font-manager/Collections.vala
+index 5e7f5cdb..43531e54 100644
+--- a/src/font-manager/Collections.vala
++++ b/src/font-manager/Collections.vala
+@@ -523,7 +523,7 @@ namespace FontManager {
+             var row = ((CollectionListRow) source.widget);
+             var drag_icon = new Gtk.Label(row.item_label.label);
+             drag_icon.add_css_class("FontManagerListRowDrag");
+-            var gtk_drag_icon = (Gtk.DragIcon) Gtk.DragIcon.get_for_drag(drag);
++            var gtk_drag_icon = new Gtk.DragIcon.get_for_drag(drag);
+             gtk_drag_icon.set_child(drag_icon);
+             return;
+         }
+diff --git a/src/font-manager/FontList.vala b/src/font-manager/FontList.vala
+index c8b170cb..9720dc2b 100644
+--- a/src/font-manager/FontList.vala
++++ b/src/font-manager/FontList.vala
+@@ -669,7 +669,7 @@ namespace FontManager {
+             widget_set_name(drag_count, "FontManagerListDragCount");
+             drag_icon.add_overlay(drag_count);
+             drag_count.set_label(selected_items.length.to_string());
+-            var gtk_drag_icon = (Gtk.DragIcon) Gtk.DragIcon.get_for_drag(drag);
++            var gtk_drag_icon = new Gtk.DragIcon.get_for_drag(drag);
+             gtk_drag_icon.set_child(drag_icon);
+             return;
+         }