Readonly Characters Are a Big Deal

Nov 10, 2025

I like Emacs UX as exemplified by Magit. I consider it to be a user interface paradigm on the same footing as UNIX pipes: An Engine for an Editor

Pipes give you 1D read-only streams of characters which excel at batch processing. Emacs is all about interactive mutable 2D buffers of attributed text.

Today I realized that an important feature of Emacs text buffers is read-only characters (manual).

Like in any editor, you can mark an entire Emacs buffer as read-only. But you also can mark individual substrings read-only, so that you can edit anywhere except specific ranges

This is a useful feature for bidirectional interaction. Consider the in-editor terminal I am using currently, which looks like this:

./zig/zig build fuzz -- message_bus = time: 3s = info(fuzz): Fuzz seed = 2355780251053186744 info(message_bus_fuzz): command weight: reserved = 0 info(message_bus_fuzz): command weight: ping = 0 info(message_bus_fuzz): command weight: pong = 0 info(message_bus_fuzz): command weight: ping_client = 0 info(message_bus_fuzz): command weight: pong_client = 0 info(message_bus_fuzz): command weight: request = 0 info(message_bus_fuzz): command weight: prepare = 37 info(message_bus_fuzz): command weight: prepare_ok = 0

The first line is the command to execute, this is typed by me manually, and then I hit a “submit” shortcut to actually run the command. Then goes the status line, which shows how long the command has been running so far and the exit code (when the command terminates). The status line is determined by the “terminal” itself. Finally, there’s output of the command itself, updated live.

In this sort of the interface, command is modifiable by the user, but is read-only for the editor. Status is the opposite — the editor updates it every second, but the user should be prevented from touching it. And the output can be CRDT-style edited by both parties (I often find it useful to edit the output in place before pasting it elsewhere).

Sadly, in VS Code I can’t prevent the user from editing the status, so my implementation is a bit janky, and this, I think, goes to the core of why I don’t see VS Code as a great platform for the kind of interactive tools I want to write.

Read-only ranges are hard to implement! Text editing hates you as is, but this feature requires tracking text attributes in an intelligent way under modifications (see sticky properties), and also feeds back into modifications themselves! No wonder Monaco, the editor engine underlying VS Code, lost this ability at some point.

Still, I feel like “does it support sticky read-only attribute?” is a good litmus test to check if an editor can support interactive applications a-la Magit seamlessly.