Lix 0.9.9:
If you click e.g. on the levels tab in the singleplayer menu there is no search button visible. But if you click at the right upper corner you can actually activate it and make it visible by that. The same works by clicking on the scrolling bar (that can be moved).
Note: The button is visible if you enter the singleplayer menu (at least if you have a level memorized by the game).
I made a video to demonstrate.
Nice catch!
Looks like it shares screen space with the directory navigation that is defined to cover most of the horizontal space. Whenever the directory navigation updates, it draws over the search button.
-- Simon
Fixed in unstable. Also translated to German the prompt in the search window to enter filenames/level titles.
-- Simon