Nowadays we can do that without JavaScript nor CSS by using a Browsers Popover API.

I am so happy to answer that question!

<button popovertarget="mypopover">Toggle the popover</button>
<div id="mypopover" popover>Popover content</div>

references:

• https://developer.mozilla.org/en-US/docs/Web/API/Popover_API
• example also https://mdn.github.io/dom-examples/popover-api/blur-background/