Sometimes the right tool is another app. MojoPad cooperates instead of competing.
Edit ▸ Edit Page in External Editor opens the current page in your text
editor — Markdown pages as .md, plain text as .txt, rich text
as .html. Every time you save there, the page updates here, live.
Pick the editor in Settings ▸ Files & Backups ▸ External apps. Press Choose… beside External text editor and your Applications folder opens: pick the app, and it is kept. You can type an app’s name instead, as it appears in your Applications folder — BBEdit, or Visual Studio Code — and it is kept as you type, even if you close Settings straight away. Left empty, pages open in the app your Mac uses for plain text, which is TextEdit unless you have changed it.
If no app on your Mac has the name you typed, MojoPad says so, names it, and opens nothing — check the spelling, or use Choose…. Something that is not an app is never used: MojoPad tells you before it makes any copy of the page.
The editor works on a copy of the page, kept in your Mac’s temporary folder and readable only by you. MojoPad deletes it when you close the window or quit. On a page with a password, or in a wiki with one, MojoPad asks before it writes that copy, because your editor may keep copies of its own that MojoPad cannot clean up. A picture you double-click is asked about the same way.
Double-click any image in a page and it opens in your image editor — Acorn, Pixelmator, Preview, whatever you set with Choose… beside Image editor (left empty, the picture opens in the app that usually opens it). Crop it, mark it up, save — the image in your page updates the moment you do. Option-double-click saves the image out to a file instead. (Sketches are the exception: double-clicking a sketch reopens MojoPad's own canvas — see Formatting.)