maybe the update of the symbol list on each notebook tab change might be a noticeable performance hit on older machines ("unfortunately" I've recently got a new notebook with a bleeding fast CPU so I won't notice :D).
That shouldn't be a problem - update of the symbol tree is performed every time the document is re-parsed while typing and if the machine is too slow, editing the document itself would be a much bigger problem already.
I think it would be even better to have filters per document, otherwise it could be confusing if I switch to another document and suddenly the symbol list is empty. After wondering about it, the user will probably realize it's because of the still set filter and then will clear the filter.
Good point, I didn't think about that.
Though having per document filters obviously requires much more efforts :(.
What about a "lazy" solution to clear the filter automatically on a tab change?