The plain-text editor has an "Insert collapsible section" button: {width=291 height=374} ... which adds this to the document: ```html <details> <summary>Click to expand</summary> </details> ``` We should automatically select the `Click to expand` text after insertion so the user can just start typing. Note that we already have this feature implemented in the plugin we add to CodeMirror(?)/Monaco(?)/whatever it is; see `app/assets/javascripts/editor/constants.js`: {width=642 height=600} That's a totally separate system to what we use for the plain-text editor; just noting we do already have support for this and it really should be consistent across editors.