This module allows Views exposed filters for taxonomy terms to show only the terms from the original result set.
For example, suppose your vocabulary has 100 terms, but the view shows only two nodes because of other filters and between them those nodes only have 6 terms. The exposed filter form for taxonomy terms will show only those 6 terms.
Enable the module, and clear your Views cache. You will get an extra option in the exposed options for the 'Taxonomy: Term' filter allowing you to limit the terms shown.
This works by running an extra query with all the filters bar the actual taxonomy term so even after filtering you still get the same list of terms. This involves doing some fairly kludgy things with the innards of Views, to wit, cloning the query object and running it separately to get the terms, and hacking these back into the form that's already been generated. Hence there is a performance consideration, and also not all aspects of the original View work (eg the pager limit).
This module is not being actively developed; patches will be reviewed.
A new maintainer or co-maintainer is needed to take this further, for example to work on changes to Views 3.x in order to make this less of a kludge.
Development of this module was sponsored by Ixis.