Manual Resize and Aspect Ratio Fit

//  This Scene has no aspect ratio lock, it will scale to fit the browser window, but will zoom to match the Game
class BackgroundScene extends Phaser.Scene
{
    gameScene;
    layer;

    constructor ()
    {
        super('BackgroundScene');
    }

    preload ()
    {
        this.load.setBaseURL('https://cdn.phaserfiles.com/v410');
        this.load.image('logo', 'assets/sprites/phaser3-logo-x2.png');
        this.load.image('fakegame', 'assets/pics/ninja-masters2.png');
    }

    create ()
    {
        this.layer = this.add.container();

        const bg = this.add.image(0, 0, 'fakegame');

        this.layer.add(bg);

        //  We launch this Scene first because we can't use `getZoom` otherwise
        this.scene.launch('GameScene');

        this.gameScene = this.scene.get('GameScene');
    }

    updateCamera ()
    {
        const width = this.scale.gameSize.width;
        const height = this.scale.gameSize.height;

        const camera = this.cameras.main;

        camera.setViewport(0, 0, width, height);

        this.layer.setPosition(width / 2, height / 2);
        this.layer.setScale(this.gameScene.getZoom());
    }

    resize ()
    {
        this.updateCamera();
    }
}

//  This Scene is aspect ratio locked at 640 x 960 (and scaled and centered accordingly)
class GameScene extends Phaser.Scene
{
    backgroundScene;
    parent;
    sizer;

    constructor ()
    {
        super('GameScene');
    }

    create ()
    {
        const width = this.scale.gameSize.width;
        const height = this.scale.gameSize.height;

        this.parent = new Phaser.Structs.Size(width, height);
        this.sizer = new Phaser.Structs.Size(640, 960, Phaser.Structs.Size.HEIGHT_CONTROLS_WIDTH, this.parent);

        this.parent.setSize(width, height);
        this.sizer.setSize(width, height);

        this.backgroundScene = this.scene.get('BackgroundScene');

        this.updateCamera();

        this.scale.on('resize', this.resize, this);

        //  Normal game stuff from here on down
        this.add.image(640 / 2, 960 / 2, 'fakegame');
    }

    updateCamera ()
    {
        const camera = this.cameras.main;

        const x = Math.ceil((this.parent.width - this.sizer.width) * 0.5);
        const y = 0;
        const scaleX = this.sizer.width / 640;
        const scaleY = this.sizer.height / 960;

        camera.setViewport(x, y, this.sizer.width, this.sizer.height);
        camera.setZoom(Math.max(scaleX, scaleY));
        camera.centerOn(320, 480);

        this.backgroundScene.updateCamera();
    }

    getZoom ()
    {
        return this.cameras.main.zoom;
    }

    resize (gameSize, baseSize, displaySize, resolution)
    {
        const width = gameSize.width;
        const height = gameSize.height;

        this.parent.setSize(width, height);
        this.sizer.setSize(width, height);

        this.updateCamera();
    }
}

const config = {
    type: Phaser.AUTO,
    backgroundColor: '#000000',
    scale: {
        mode: Phaser.Scale.RESIZE,
        parent: 'game-container',
        width: 640,
        height: 960,
        min: {
            width: 320,
            height: 480
        },
        max: {
            width: 1920,
            height: 1400
        }
    },
    scene: [ BackgroundScene, GameScene ]
};

const game = new Phaser.Game(config);