Personally I prefer the solution of providing a JS interface for gadgets. However, I think a cm prototype method + ext.CodeMirror.ready hook will be better than an additional hook.

Most users use scripts/gadgets for that sort of thing, see for example https://ru.wikipedia.org/wiki/Участник:DonRumata/Gadget-keybLayout.js
I think, since projects’ needs on this can be very different, this is better served by providing some access to extending CodeMirror in whichever way is easier. To clarify, it is specifically a common problem for wikis in Cyrillic that some users would (un)knowingly mix up the layouts in the same words, so just extending specialCharsExtension would probably not work as well for that use case since any hypothetical code would need to trigger on Пpимep (p/e are Latin) and not trigger on Example.

Since we have enabled linters in CodeMirror now, how about highlighting these characters as warnings? The interface will be much easier for us to provide and probably also more flexible for user script authors. For example, a user script can pass an arbitrary linter function ( content: string ) => Diagnostic[] | Promise<Diagnostic[]> to a prototype method cm.customLint( linter ), and here is the signature of interface Diagnostic. In this way, the user script can even suggest quick-fix actions.

In T383285#11731905, @Bhsd wrote:

Since we have enabled linters in CodeMirror now, how about highlighting these characters as warnings? The interface will be much easier for us to provide and probably also more flexible for user script authors. For example, a user script can pass an arbitrary linter function ( content: string ) => Diagnostic[] | Promise<Diagnostic[]> to a prototype method cm.customLint( linter ), and here is the signature of interface Diagnostic. In this way, the user script can even suggest quick-fix actions.

That sounds great! And I see from Discord we had another user who was content with a user script/gadget solution :)

In addition to adding the new public customLint() method, we could perhaps go ahead and implement one gadget for a select wiki, and publish that code in our documentation (say at Help:Extension:CodeMirror/Custom linters) so other wikis and individual users can borrow from it to craft their own implementation.

In T383285#11756597, @MusikAnimal wrote:

And I see from Discord we had another user who was content with a user script/gadget solution :)

I think the request from Discord is related to bracket matching instead, for which I just submitted a patch.

You're right. Sorry, I misread!

My changes to the task description still apply though, I think – no need to bring preferences into this.

Change #1261901 had a related patch set uploaded (by Bhsd; author: Bhsd):

[mediawiki/extensions/CodeMirror@master] CodeMirror: applyLinter() method

https://gerrit.wikimedia.org/r/1261901

Change #1261901 merged by jenkins-bot:

[mediawiki/extensions/CodeMirror@master] CodeMirror: applyLinter() method

https://gerrit.wikimedia.org/r/1261901

In T383285#10450479, @stjn wrote:

see for example https://ru.wikipedia.org/wiki/Участник:DonRumata/Gadget-keybLayout.js

The mentioned script can be converted to a CodeMirror extension like this (after the deployment next week):

const cyrPattern = '[\u0401-\u040F\u0410-\u042F\u0430-\u044F\u0451-\u045F\u0460-\u0486\u0490-\u04CC\u04D0-\u04F9]';
const latPattern = '[A-Za-zªºÀ-ÖØ-öø-ʸˠ-ˤᴀ-ᴥᴬ-ᵜᵢ-ᵥᵫ-ᵷᵹ-ᶾḀ-ỿⁱⁿₐ-ₜℲⅎⅠ-ↈⱠ-ⱿꜢ-ꞇꞋ-ꞿꟂ-ꟊꟵ-ꟿꬰ-ꭚꭜ-ꭤꭦ-ꭩff-stＡ-Ｚａ-ｚ]';
const pattern = new RegExp( `${ latPattern }+(?=${ cyrPattern })|(${ cyrPattern }+)${ latPattern }+`, 'g' );
mw.hook( 'ext.CodeMirror.ready' ).add( ( cm ) => {
	cm.applyLinter( {
		pattern,
		callback( mt ) {
			return {
				from: mt.index + ( mt[ 1 ] || '').length,
				to: mt.index + mt[ 0 ].length,
				severity: 'warning',
				message: 'Latin letters among Cyrillic letters'
			};
		}
	} );
} );

In T383285#11756597, @MusikAnimal wrote:

we could perhaps go ahead and implement one gadget for a select wiki, and publish that code in our documentation (say at Help:Extension:CodeMirror/Custom linters) so other wikis and individual users can borrow from it to craft their own implementation.

An ongoing discussion on EN Wikisource that may be relevant: https://en.wikisource.org/wiki/Wikisource:Scriptorium#c-Nosferattus-20260406000700-Look-alike_Cyrillic_characters

In T383285#11756597, @MusikAnimal wrote:

In addition to adding the new public customLint() method, we could perhaps go ahead and implement one gadget for a select wiki, and publish that code in our documentation (say at Help:Extension:CodeMirror/Custom linters) so other wikis and individual users can borrow from it to craft their own implementation.

Hi! Do you still plan to proceed with an example gadget, or can I close this as the requested task has been completed?

In T383285#11870452, @Bhsd wrote:

Hi! Do you still plan to proceed with an example gadget, or can I close this as the requested task has been completed?

Sorry for the late reply! No, I don't plan to build a gadget unless requested. If/when that happens, I can link to it in the docs on mediawiki.org.