As a first step towards implementing the tags portion of T159942: Explore designs for including filtering for users, edit tags, and namespaces, replace the "tag filter" input field with a widget that allows you to look up and select multiple tags.
You should be able to look up tags both by their internal name and their display name (see "Tag name" and "Appearance on change lists" respectively on Special:Tags). This is important because the current input only recognizes the internal names, but the interface uses display names everywhere. Also, the internal names for software-defined tags are always in English, which is a problem for non-English-speaking wikis (AbuseFilter-defined tags, however, generally will have an internal name in the local language, which means this isn't even consistent).
Selections of multiple tags will be ORed, not ANDed, for the time being, because that's easier to implement.
Screenshots from plwiki that illustrate this problem:
