Source: WindowLayer.js

//-----------------------------------------------------------------------------
/**
 * The layer which contains game windows.
 *
 * @class
 * @extends PIXI.Container
 */
function WindowLayer() {
    this.initialize(...arguments);
}

WindowLayer.prototype = Object.create(PIXI.Container.prototype);
WindowLayer.prototype.constructor = WindowLayer;

WindowLayer.prototype.initialize = function() {
    PIXI.Container.call(this);
};

/**
 * Updates the window layer for each frame.
 */
WindowLayer.prototype.update = function() {
    for (const child of this.children) {
        if (child.update) {
            child.update();
        }
    }
};

/**
 * Renders the object using the WebGL renderer.
 *
 * @param {PIXI.Renderer} renderer - The renderer.
 */
WindowLayer.prototype.render = function render(renderer) {
    if (!this.visible) {
        return;
    }

    const graphics = new PIXI.Graphics();
    const gl = renderer.gl;
    const children = this.children.clone();

    renderer.framebuffer.forceStencil();
    graphics.transform = this.transform;
    renderer.batch.flush();
    gl.enable(gl.STENCIL_TEST);

    while (children.length > 0) {
        const win = children.pop();
        if (win._isWindow && win.visible && win.openness > 0) {
            gl.stencilFunc(gl.EQUAL, 0, ~0);
            gl.stencilOp(gl.KEEP, gl.KEEP, gl.KEEP);
            win.render(renderer);
            renderer.batch.flush();
            graphics.clear();
            win.drawShape(graphics);
            gl.stencilFunc(gl.ALWAYS, 1, ~0);
            gl.stencilOp(gl.REPLACE, gl.REPLACE, gl.REPLACE);
            gl.blendFunc(gl.ZERO, gl.ONE);
            graphics.render(renderer);
            renderer.batch.flush();
            gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
        }
    }

    gl.disable(gl.STENCIL_TEST);
    gl.clear(gl.STENCIL_BUFFER_BIT);
    gl.clearStencil(0);
    renderer.batch.flush();

    for (const child of this.children) {
        if (!child._isWindow && child.visible) {
            child.render(renderer);
        }
    }

    renderer.batch.flush();
};