Org-Mode and Leo Editor have a collapsible outline view like that, but while you can use them to author documents, it's obviously hard to compare them to LyX.
Org-mode is certainly not directly comparable. But the output you get from Org-mode is comparable. Org-mode has excellent support for seamless export to LaTeX and LaTeX-generated-PDF, and many use it for writing, e.g., academic papers. Output is as beautiful as any other LaTeX tool, depends on the styling you build in. Here, e.g., is someone who modified LaTeX output to have an "Edward Tufte" look, once you incorporate the mods n your Emacs init file, conversion to this output happens automatically from the plain text Org file export:
https://damitr.org/2014/01/09/latex-tufte-class-in-org-mode/
Org-mode is certainly not directly comparable. But the output you get from Org-mode is comparable. Org-mode has excellent support for seamless export to LaTeX and LaTeX-generated-PDF, and many use it for writing, e.g., academic papers. Output is as beautiful as any other LaTeX tool, depends on the styling you build in. Here, e.g., is someone who modified LaTeX output to have an "Edward Tufte" look, once you incorporate the mods n your Emacs init file, conversion to this output happens automatically from the plain text Org file export: https://damitr.org/2014/01/09/latex-tufte-class-in-org-mode/