At the moment it is very difficult to select a category with a long name.
In the example below, I was looking for "Série télévisée de déroulant en Angleterre". There is hopefully a pop-up when you hover the name of the category, but it takes time to hover all categories (and finally sometimes figure that's not the ones you were expecting - see T187342).
Tested on Chromium and Firefox, last versions.
Update: I notice that typing a few words, then click outside, then click again in the search field makes makes the dropdown bigger and visible.