Very good idea.
An example implementation is the filtering string in the History sidebar in the Firebox, another example is the filtering principle in 'Everything'.
Search among tags is absolutely required. I have several hundreds of them, and I need to wait looong seconds for the alphabetic list to scroll.
May I add that a full-blown tag management window is also required. Especially since we have hierarchical tags. I often find myself adjusting categories when filing a note : discovering redundant tags, obsolete tags, tags that should be merged or split up, and even tags that are the result of typing mistakes.
Currently, the task has to be done with closed eyes, so to speak.
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