The user input widget on Special:Block (used for the form's target field) was replaced with a users multiselect in T324602.

The reasoning behind this was:

• Special:Block updates according to the value of the target field
• The user input widget does not differentiate between accepted input vs in-progress, whereas the multiselect does

There were problems with this however:

• (Conceptually) It's strange to use a multiselect with limit 1
• (Practically) This bug: T334711: TagMultiselectWidget value is uneditable once the maximum number of tags are entered

A few problems were reported and discussed, e.g. on T332994: Allow target field to be editable on Special:Block if pre-filled with an IP address. In particular, T332994#8780051 makes the widget very troublesome to use, without fixing T334711.

The quickest way to solve this is to restore the user input widget.

Note: The temporary account handling added to CheckUser in T324602 also needs updating.