Function isFullscreenElement

src/js/tether.js:35–43

(e)

const TetherBase = { modules: [Constraint, Abutment, Shift] };

function isFullscreenElement(e) {
  let d = e.ownerDocument;
  let fe =
    d.fullscreenElement ||
    d.webkitFullscreenElement ||
    d.mozFullScreenElement ||
    d.msFullscreenElement;
  return fe === e;
}

function within(a, b, diff = 1) {
  return a + diff >= b && b >= a - diff;