Hi! When you use the folder tree filter all the expanded folders are collapsed. If you have hundred of expanded folders you're forced to re-open them manually, one by one. This is a regression compared to KMail 1.
I can confirm it :( Don't know how to fix it for the moment
I don't know the technical details, but what about storing the tree structure when you start typing (or paste a string) into the filter and restore after clearing (with backspace / cancel / clear button) the filter search input ???
Fixed in master/4.7.1
Thanks Laurent!