i3-sway: allow inverse_outer on smart_gaps

Change the boolean `smartGaps` option to an `enum` that expects common
values of i3 and sway ("on" "off" "inverse_outer").

modules/misc/news/2026/02/2026-02-24_19-17-29.nix

@@ -0,0 +1,9 @@
+{
+  time = "2026-02-24T18:17:29+00:00";
+  condition = true;
+  message = ''
+    The options 'xsession.windowManager.i3.config.gaps.smartGaps' and
+    'wayland.windowManager.sway.config.gaps.smartGaps' now expects either "on",
+    "off" (default) or "inverse_outer".
+  '';
+}

modules/services/window-managers/i3-sway/lib/functions.nix

@@ -189,7 +189,7 @@ rec {
           (optionalString (bottom != null) "gaps bottom ${toString bottom}")
           (optionalString (left != null) "gaps left ${toString left}")
           (optionalString (right != null) "gaps right ${toString right}")
-          (optionalString smartGaps "smart_gaps on")
+          (optionalString (smartGaps != "off") "smart_gaps ${smartGaps}")
           (optionalString (smartBorders != "off") "smart_borders ${smartBorders}")
         ]
     );

modules/services/window-managers/i3-sway/lib/options.nix

@@ -932,13 +932,27 @@ in
           };
 
           smartGaps = mkOption {
-            type = types.bool;
-            default = false;
+            type = types.either types.bool (
+              types.enum [
+                "on"
+                "off"
+                "inverse_outer"
+              ]
+            );
+            apply =
+              value:
+              if value == true then
+                "on"
+              else if value == false then
+                "off"
+              else
+                value;
+            default = "off";
             description = ''
               This option controls whether to disable all gaps (outer and inner)
               on workspace with a single container.
             '';
-            example = true;
+            example = "on";
           };
 
           smartBorders = mkOption {