input: Multi cursors

[zanmato]

Closes #879

Description

This adds multi cursors to the input, alt-click and shift-alt drag to add multiple. I've been running this for a while in my fork, with another undo manager. I've now pulled it out and rebased it onto the new undo manager.

I know this is too much to review, but it would be nice to get it upstream. I am putting this here as a draft if you want to use it as a basis for your own implementation, also the tests are pretty good to keep if you do decide to add it.

Video

Screencast.From.2026-08-26.10-55-53.mp4

How to Test

Ran the editor example

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[huacnlee]

In video, highlgihts broken after text changed.

[zanmato]

In video, highlgihts broken after text changed.

Hmm, I don't see it. Where do you mean? It looks the same after undoing the invalid syntax

[huacnlee]

Oh, never mind that. I was wrong. It's correct.

[huacnlee]

Hi @zanmato please keep going when you have time, I'd like merge this feature.

[zanmato]

Hi @zanmato please keep going when you have time, I'd like merge this feature.

I've rebased it with the new affinity changes. Note that there is a performance issue with undoing a lot of cursors, because of parse_injection_layers. This issue is in main as well, but gets amplified with a lot of cursors. I didn't fix it here because there's already a lot of changes 😃