diff --git a/hosts/common/programs/mpv/default.nix b/hosts/common/programs/mpv/default.nix
index a7101bdbb..d4730f8e6 100644
--- a/hosts/common/programs/mpv/default.nix
+++ b/hosts/common/programs/mpv/default.nix
@@ -116,6 +116,12 @@ let
         --replace-fail \
           "mp.commandv('cycle', 'mute')" \
           "mp.set_property_bool('user-data/sane-sysvol/mute', not mp.get_property_bool('user-data/sane-sysvol/mute'))"
+
+      # tweak the top-bar "maximize" button to actually act as a "fullscreen" button.
+      substituteInPlace src/uosc/elements/TopBar.lua \
+        --replace-fail \
+          'get_maximized_command,' \
+          '"cycle fullscreen",'
     '';
   });
   mpv-unwrapped = pkgs.mpv-unwrapped.overrideAttrs (upstream: {