How Can I Copy Text From Hidden Textarea to Clipboard?

You can create a temporary input element that is appended to the body, set it its value to the textarea's content and then use it for the copy function. Then you remove it from the dom. This will work - irrespective of the display state of the text area.

Note that I did not create this method - I got it from somewhere (possibly another SO answer) and have used it since in a number of instances.

function copyContents() {
  var $temp = $("<input>");
  var content = $('#element_html').text();
  
	$("body").append($temp);
    $temp.val(content).select();
    document.execCommand("copy");
    $temp.remove();
}

#element_html {
display: none
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<textarea id="element_html">Hidden textarea content</textarea>
<button onclick="copyContents()">Click to copy</button>

<input type="text" placeholder="Paste here">

opacity: .01;

does the job. You should combine it with:

height:0;
position:absolute;
z-index: -1;

Do not use pointer-events:none; as it will stop .select() from working, as well.

function copyContents() {
  $('#element_html').select();
  document.execCommand('copy');
}

#element_html {
  position: absolute;
  opacity: .01;
  height: 0;
  overflow: hidden;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<textarea id="element_html">Which textarea?</textarea>
<button onclick="copyContents()">Copy</button>

<input type="text" placeholder="Paste it here...">