In the “All categories” dropdown input, if you hover your mouse over any category title it will display a tooltip containing the category’s description.
However, if you hover your mouse over the counter beside the category title, the tooltip will also contain any HTML in the description + the ellipsis HTML entity.
This can be reproduced on this forum, the Twitter community forum, and other forums.
The contents of this tooltip come from the title attribute on the li tag in the dropdown.