WebGL 2.0. Scaling on mouse click

HTML

<canvas id="renderCanvas" width="250" height="250"></canvas>

<script src="https://cdnjs.cloudflare.com/ajax/libs/gl-matrix/2.4.0/gl-matrix-min.js"></script>

CSS

#renderCanvas {
    border: 5px solid #e0aeae;
}

JavaScript

var vertexShaderSource =
    `#version 300 es
    
    in vec2 a_Position;
    uniform mat4 u_ModelMatrix;

    void main()
    {
        gl_Position = u_ModelMatrix * vec4(a_Position, 0.0, 1.0);
    }`;

var fragmentShaderSource =
    `#version 300 es
    
    precision mediump float;
    uniform vec3 u_Color;
    out vec4 fragColor;

    void main()
    {
        fragColor = vec4(u_Color, 1.0);
    }`;

var canvas = document.getElementById("renderCanvas");
var gl = canvas.getContext("webgl2");

var vShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vShader, vertexShaderSource);
gl.compileShader(vShader);

var fShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fShader, fragmentShaderSource);
gl.compileShader(fShader);

var program = gl.createProgram();
gl.attachShader(program, vShader);
gl.attachShader(program, fShader);
gl.linkProgram(program);
gl.useProgram(program);

var vertices = new Float32Array([
    -0.5, 0.5, -0.5, -0.5, 0.5, 0.5, 0.5, -0.5
]);

var vbo = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vbo);
gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

var a_Position = gl.getAttribLocation(program, "a_Position");
gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(a_Position);

var u_Color = gl.getUniformLocation(program, "u_Color");
gl.uniform3f(u_Color, 0.207, 0.635, 0.125);

var modelMatrix = mat4.create();
var u_ModelMatrix = gl.getUniformLocation(program, "u_ModelMatrix");

var maxScale = 1.5;
var minScale = 0.5;
var currentscale = minScale;
var scaleStep = 0.1;

draw(currentscale);

canvas.onmousedown = function ()
{
    currentscale += scaleStep;

    if (currentscale > maxScale)
    {
        scaleStep *= -1;
        currentscale = maxScale;
    }
    else if (currentscale < minScale)
    {
        scaleStep *= -1;
        currentscale = minScale;
    }

    draw(currentscale);
}

function draw(scale)
{
    mat4.identity(modelMatrix);
    mat4.scale(modelMatrix,...