PixelPerfectScaleMode is a scaling mode which maintains the game's aspect ratio.
When you shrink or grow the window, the width and height of the game will adjust,
either scaling the game or adding black bars as needed.
However, it will only scale the game by integer factors, to maintain pixel perfect rendering. This may cause the game window to be windowboxed on all sides, scaling the game up only when there is enough room to exactly double the render scale.
To enable it in your project, use FlxG.scaleMode = new PixelPerfectScaleMode();.