There are endless amount of outliner programs available. The subject has emerged since the 1980s. One of the most interesting example is the zim-desktop wiki. From the graphical layout it works with the established two pane model. In the left window there are sections available while the right larger window shows the content of a section.
In contrast to other outliner tools like cherrytree, the difference is, that zimwiki is using the underlying file system. Instead of creating a single XML file which holds the database content, zimwiki stores the information into an entire directory. The advantage is, that external files like images and very important attachments can be stored in this way. For example, it is possible to create a new zimwiki entry and then append an Excel sheet to this entry.
One disadvantage of the project is, that it was realized in the python language. Python is a great choice for prototyping an app and for scientific purposes but it fails for writing productive code. The result are many unfixed bugs in the zimwiki software and the only way to overcome it is to rewrite the entire program in the C/C++ software which is the industry standard for production ready software.
Despite the disadvantages, zimwiki can be recommended because it shows how future outliner tools have to look like. The amount of features is not overwhelming and the preference of the markdown format as internal text formatting makes sense.