FWIW, I have used single panel outlines for years starting with Omni Outliner on my OSX 10.0.1 days. However, after switching off Mac, I've come to depended on Ecco Pro[1] which I presently use on Ubuntu/Gnome via Wine.
Ecco Pro only has one level of undo but it just does just what is needed and I've never had it loose any data on me. It was last compiled in 1997 but there are ways to tweak it out for modern OS's.
I have automated scripts to convert Ecco outlines into Markdown also have time invoicing outputs etc. As Ecco does not have any rich text or any intrinsic special formatting coding, its easy to use Ecco as your own personal note taking DSL. For Example: .: <title_block> :. [.] - todo (open) [x] - todo (done) [~] - todo (in progress) etc...