Netboot related buttons are not clickable in Safari and Firefox on OS X

According one ticket on support, user reports that buttons in netboot & subordinates tabs are not clickable in Safari browser.

It is not possible either on MacBook or iPad. Firefox on windows machine works fine.

Reproducible on Safari (Safari/13.0.5 Webkit/605.1.15) and Firefox (v74) on Mac OS X. Chrome on OS X works fine.

Probable cause

Detection of clicked button by document.activeElement, which doesn't work reliable across different browsers.

For example, on macOS systems, elements that aren't text input elements are not typically focusable by default.

https://developer.mozilla.org/en-US/docs/Web/API/DocumentOrShadowRoot/activeElement

Button events handling is at line 80.

https://gitlab.labs.nic.cz/turris/foris/foris-subordinates-plugin/-/blob/master/foris_plugins/subordinates/templates/subordinates/subordinates_setup.html.j2#L80