Also, when reading long paragraphs, some users, like me, select a line or word to mark the place before scrolling to top to look something up, change tabs and come back later, etc.

Also, it collides with other "show popup on selection" extensions, like translators, for example, which show a popup and translate the selected word immediately.

A timeout would be in place; eg, under options > Search Popup > timeout = 5 would make the popup disappear after 5 seconds.