With current josm-latest builds, JOSM’s built-in level selector has been improved with several features which help when using the editor for indoor mapping, especially in more complex buildings. For example, it now supports repeat_on, allows more than one level to be visible at a time (e.g. for drawing steps between levels), and lets you choose if you want to hide inactive elements entirely for a less crowded view.
Thanks to Dirk for reviewing and merging #24482. 