Make Writ the app for .md files

Once, per file type. After that a double-click in Finder or Explorer opens the file in Writ, rendered.

macOS

1. Open Settings (⌘,), Files. Each group has its own row with a Make default button: Markdown, Plain text & logs, Config & data, Source code. Press it for the groups you want; the row then reads "Writ is the default".
2. macOS keeps the choice per type, so claiming Markdown does not change what opens .txt. If a row will not take, Finder's Get Info on one file, Open with: Writ, Change All does the same thing by hand.

Windows

1. Right-click any .md file, Open with, Choose another app.
2. Pick Writ and tick "Always use this app".
3. Repeat for .txt or any other extension you want here.

Linux

The .deb installs a desktop entry named Writ.desktop, so Writ appears in your file manager's Open With list. To make it the default from a terminal:

xdg-mime default Writ.desktop text/markdown
xdg-mime default Writ.desktop text/plain

With the AppImage, register it first with AppImageLauncher (or any integrator that writes a desktop entry), then run the same commands with the name it created.

What opening a file means

The file opens in a tab under its own name. Edits stay in Writ until ⌘S writes them back, and if the file changes on disk while it is open the status bar says so first. Closing the tab never deletes or changes the file. The note also goes into history, so ⌘⇧F finds it later even if the file has moved.

Related: the default app in the docs.