Hard to believe, but I think fullscreen truly works, even through sleep.

desktop/src-common/advent/screens/inventory.clj

@@ -10,6 +10,7 @@
             [advent.utils :as utils])
   (:import [com.badlogic.gdx.graphics Pixmap Pixmap$Filter Texture Texture$TextureFilter]
            [com.badlogic.gdx.graphics.g2d TextureRegion]
+           [com.badlogic.gdx.utils.viewport FitViewport]
            [com.badlogic.gdx.scenes.scene2d.utils Align]
            [com.badlogic.gdx Application Audio Files Game Gdx Graphics Input
             InputMultiplexer InputProcessor Net Preferences Screen]))
@@ -45,7 +46,8 @@
        :highlighted-text highlighted-text}))
 
   :on-render
-  (fn [screen [entities]]
+  (fn [{:keys [^FitViewport viewport] :as screen} [entities]]
+    (.apply viewport)
     (let [
           entities (utils/apply-tweens screen entities (:tweens entities))
           opacity (get-in entities [:opacity])