Compute the maximum available canvas width and height in a browser

max_canvas_size.js

async function maxCanvasSize(ratio) {
    let cvs = document.createElement("canvas");
    let xmin = 1, xmax = 1 << 18;
    while (xmax - xmin > 1) {
        let x = ((xmin + xmax) / 2) | 0;
        let y = (x / ratio) | 0;
        if (await canvasTest(cvs, x, y)) xmin = x;
        else xmax = x;
    }
    return [xmin, xmin / ratio | 0];
}

function canvasTest(cvs, width, height) {
    return new Promise(a => {
        try {
            cvs.width = width;
            cvs.height = height;
            let ctx = cvs.getContext("2d");
            ctx.fillRect(width - 1, height - 1, 1, 1);
            return a(!!ctx.getImageData(width - 1, height - 1, 1, 1).data[3]);
        }
        catch (e) {
            return a(false);
        }
    });
}