feat: Add a setting that automatically launches the game in fullscreen

source/funkin/Preferences.hx

@@ -157,6 +157,25 @@ class Preferences
     return value;
   }
 
+  /**
+   * If enabled, the game will automatically launch in fullscreen on startup.
+   * @default `true`
+   */
+  public static var autoFullscreen(get, set):Bool;
+
+  static function get_autoFullscreen():Bool
+  {
+    return Save?.instance?.options?.autoFullscreen ?? true;
+  }
+
+  static function set_autoFullscreen(value:Bool):Bool
+  {
+    var save:Save = Save.instance;
+    save.options.autoFullscreen = value;
+    save.flush();
+    return value;
+  }
+
   public static var unlockedFramerate(get, set):Bool;
 
   static function get_unlockedFramerate():Bool
@@ -211,6 +230,8 @@ class Preferences
     #if web
     toggleFramerateCap(Preferences.unlockedFramerate);
     #end
+    // Apply the autoFullscreen setting (launches the game in fullscreen automatically)
+    FlxG.fullscreen = Preferences.autoFullscreen;
   }
 
   static function toggleFramerateCap(unlocked:Bool):Void

source/funkin/save/Save.hx

@@ -97,6 +97,7 @@ class Save
           zoomCamera: true,
           debugDisplay: false,
           autoPause: true,
+          autoFullscreen: false,
           inputOffset: 0,
           audioVisualOffset: 0,
           unlockedFramerate: false,
@@ -1339,6 +1340,12 @@ typedef SaveDataOptions =
    */
   var autoPause:Bool;
 
+  /**
+   * If enabled, the game will automatically launch in fullscreen on startup.
+   * @default `true`
+   */
+  var autoFullscreen:Bool;
+
   /**
    * Offset the user's inputs by this many ms.
    * @default `0`

source/funkin/ui/options/PreferencesMenu.hx

@@ -124,6 +124,10 @@ class PreferencesMenu extends Page
       Preferences.framerate = Std.int(value);
     }, null, Preferences.framerate, 30, 300, 5, 0);
     #end
+
+    createPrefItemCheckbox('Launch in Fullscreen', 'Automatically launch the game in fullscreen on startup', function(value:Bool):Void {
+      Preferences.autoFullscreen = value;
+    }, Preferences.autoFullscreen);
   }
 
   override function update(elapsed:Float):Void