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  <input>

  <button>CLICK</button> <button>MOUSEUP</button> <button>MOUSEDOWN</button> <button>MOUSEDOWN 2</button>

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document.querySelector('button').addEventListener('click', function () {
  document.querySelector('input').focus();
}, false);

document.querySelectorAll('button')[1].addEventListener('mouseup', function () {
  document.querySelector('input').focus();
}, false);

document.querySelectorAll('button')[2].addEventListener('mousedown', function (e) {
  document.querySelector('input').focus();
}, false);

document.querySelectorAll('button')[3].addEventListener('mousedown', function (e) {
  document.querySelector('input').focus();
  e.preventDefault();
}, false);