> I do use Shift-Left/Right to adjust the extension width, but I was
> assuming ZTree had switched modes to spell search, and so I was
> concentrating on the spell search string and not looking at the file
> window.

The whole point of Active spell search is that there is no change of 'mode' per say - just additional shift-letter support that is caught to set the search string, rather than do the normal dir/file command.
So there is no easy way to support both the existing extension width adjustment and the proposed search editing.
Given that '|' provides full editing capabilities already, this just seems like major feature creep, sorry.