Recently, I've found that I have too many folders to browse the tree structure manually and I have activated the search box. I've been using this mainly for sorting emails - moving (by mouse drag) resolved issues from the inbox, which I use as a kind of ToDo list, to the appropriate "archive" folder.
You may find this workflow obscure, but it has worked for me for years. Now, right after simplifying it with the search box, the first update has broken it - when I start typing to search, the contents of the selected folder (the inbox) gets hidden, so I don't see the message which I want to drag to the destination folder, thus I cannot move it.

Reproducible: Always

Steps to Reproduce:
1. activate quicksearch field in folder list
nastavení (settings) => vzhled (appearance?) => uspořádání (layout?) => seznam složek/zobrazovat pole pro rychlé hledání (folder list/show quicksearch?)
2. select some folder that is not empty so that you see list of messages in that folder
3. select some message from that folder so that you see the contents of the message
4. start typing in the folder quicksearch box
Actual Results:  
the messages shown and the contents of the selected message disappear

Expected Results:  
the contents of the selected folder & message are shown until you explicitly select different ones (e.g. by clicking on something that has been found via the quicksearch)