In SourceTree 3.4.23, the interactive rebase window has lost the buttons bar. While some actions are still available as context menu, the missing "OK" and "Cancel" buttons make it almost impossible to use interactive rebase anymore. Downgrading to 3.4.22 solved the problem.
Bottom part of the interactive rebase window in version 3.4.23 (broken):

Bottom part of the interactive rebase window in previous versions (OK):
