mouseDown (evt, mouseTarget, startX, startY) {
      const pt = screenToPt(startX, startY)

      textinput.focus()
      setCursorFromPoint(pt.x, pt.y)
      lastX = startX
      lastY = startY

      // TODO: Find way to block native selection
    }
@param {external:MouseEvent} evt
@param {Element} mouseTarget
@param {Float} startX
@param {Float} startY
@returns {void}