I've also noticed this on OSX when working with filenames that contain non-ascii characters. It happens even when using the default theme.
The issue is that OSX stores file paths in UTF-8 NFD normalization where latin characters with diacritics are formed via the latin character, a combining character and then the diacritic character.
For a plugin I was working on, I used the unicodedata python module and ended up calling:
That said, I'm pretty sure Jon must be aware of what is causing this and the solution since the sidebar is well-behaved with these filenames.