Before disabling the browser's autocomplete function for tags (MBS-11367), the genre suggestions should not only include custom tags of the user, but also show them as first results to allow efficient tagging according to the user's own preferences.

Another very important feature of the browser autocomplete is the ability to delete entries from the suggestions, which is very useful to remove typos and/or tag name variations to prevent them from not showing up again. This could be handled by only suggesting those custom tags for which the user currently has tagged entities, so typos and name variations which are no longer in use will not be shown.

(Adapted from my comment on MBS-11367 and reosarevok's suggestion)

TL;DR the resulting list of tag suggestions should show the following (in the given order):

1. Genres and custom tags which are used by the active user, ordered by frequency
2. Other genres, ordered by name