Use the expanded filter editor by default
instead of having the scroll sideways and find a "..." button, just use the expanded layout by default. it's not that complex and the interface is a lot cleaner and less buggy.
In a lot of cases (actually all cases for my users) the default compact filter entry has issues where lot of folders or long folder names cause display issues, like not showing the whole folder list box, and placing the "..." buttons right over the middle of the list box. the compact box is fairly unusable once you have a few subfolders in your mailbox.