Contest Timer Wrong

[slightlyskepticalpotat]

The contest timer is sometimes drastically wrong, but returns to normal after a refresh. I think this is due to chromium throttling javascript.

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Platform: Chromium 91.0.4472.114 (64-bit) on Ubuntu 20.04.02

Steps to reproduce:

• Join a timed contest
• Open two tabs with timers
• Stay on one for an hour

The timer on the focused tab should be correct, and the timer on the other tab should be an hour out of date. After refreshing, the delayed timer should go back to normal.

[slightlyskepticalpotat]

Looks like it's because of setInterval being wack here https://github.com/DMOJ/online-judge/blob/3f5212fdcbfce92fe28f8d43faf2e9b31a4afc00/resources/common.js#L181

[slightlyskepticalpotat]

What if I just changed it into

var timer = setInterval(function () {
    if (!document.hidden){ // addition to prevent it updating when tab not active
        var time = Math.round(initial - (Date.now() - start) / 1000);
        if (time <= 0) {
            clearInterval(timer);
            setTimeout(function() {
                window.location.reload();
            }, 2000);
        }
        var d = Math.floor(time / 86400);
        var h = Math.floor(time % 86400 / 3600);
        var m = Math.floor(time % 3600 / 60);
        var s = time % 60;
        if (d > 0)
            label.text(npgettext('time format with day', '%d day %h:%m:%s', '%d days %h:%m:%s', d)
                .replace('%d', d).replace('%h', format(h)).replace('%m', format(m)).replace('%s', format(s)));
        else
            label.text(pgettext('time format without day', '%h:%m:%s')
                .replace('%h', format(h)).replace('%m', format(m)).replace('%s', format(s)));
    }}, 1000);

[slightlyskepticalpotat]

It looks cursed but it should work. Will PR after I test.