// ==UserScript==
// @name        ⌘⌫ Element
// @description ⌘⌫ deletes element under cursor, ⌘Z puts back
// @match       *://*.*
// ==/UserScript==

let undoWindow        = 3000 // can undo for this long
let undoCooldown      = 1500 // will prevent default undo for this long after last undo
let scheduledDeletion = null // timeout id for deleteElements
let lastHide          = 0    // timestamp for last element was hidden
let lastUndo          = 0    // timestamp for last successful undo
let hiddenElements    = []   // elements that can be unhidden by undo

let cmdCombo       = (ev, key) => !ev.shiftKey && !ev.ctrlKey && !ev.altKey && ev.metaKey && ev.key == key
let tsWithin       = (ts, t)   => (Date.now() - ts) <= t
let deleteElements = ()        => { let e; while ([e,] = hiddenElements.pop()||[]) e.parentNode.removeChild(e) }

document.addEventListener('keydown', function(ev) {

  if (cmdCombo(ev, 'Backspace')) {
    let el = [...document.querySelectorAll(":hover")].pop() // get innermost elem with under pointer
    hiddenElements.push([el, el.style.display])             // keep track of it for undo
    el.style.display = 'none'                               // hide it
    lastHide = Date.now()                                   // timestamp for undo window window
    clearTimeout(scheduledDeletion)                         // clear pending deletion to extend undo window
    scheduledDeletion = setTimeout(deleteElements, undoWindow + 1) // really delete elements after undo window times out
    ev.preventDefault()
  }

  if (cmdCombo(ev, 'z') && tsWithin(lastHide, undoWindow)) {
    let [el, d] = hiddenElements.pop() || []
    if (el) {
      el.style.display = d
      lastUndo = Date.now()
    }
    // preventDefault if we restored an element, or if it's only been 1500ms since last undo,
    // so you don't accidentally reopen a tab by way of over-undoing
    if (el || tsWithin(lastUndo, undoCooldown)) ev.preventDefault()
  }

})