Thanks for taking the time to report this!

What is the exact difference to T85787 ? Or do they "just" depend on each other?

Bug T85787 is that CharInsert uses insertTags() which is marked deprecated which gives warning messages. This is not a big problem until the deprecated function is removed, but I think it is nice to avoid warning messages.

This bug (T88922) is that insertTags() and its replacement mw.toolbar.insertTags() are defined in the module mediawiki.toolbar. Most users use the user preference "Show edit toolbar" which I think loads this module. But users who are not using the toolbar get error message "ReferenceError: insertTags is not defined" and no character is inserted when they try to use CharInserts.

The mediawiki.toolbar module doesn't initialise the toolbar, it provides methods for interacting with the editing area. Essentially the tools needed for something else to make a basic toolbar.

If Charset uses these methods, it should simply declare a dependency on this module to ensure it is loaded. Right now it relies on MediaWiki core providing the classic toolbar and it loading these methods.

Looking deeper, CharInsert really needs maintenance work. It is still using inline event handlers nested in HTML:

function charInsertChar( $start, $end = '' ) {
	..
	return Xml::element( 'a',
		array(
			'onclick' => "insertTags('$estart','$eend','');return false",
			'href'    => "javascript:void()" ),
		$inline );
}

The error

ReferenceError: insertTags is not defined
SyntaxError: expected expression, got ')'

also appears when users click on any link at
https://ro.wikipedia.org/wiki/Special:Upload

After the page is loaded, mw.loader.getState('mediawiki.toolbar') returns "registered". I only get

Use of "insertTags" is deprecated. Use mw.toolbar.insertTags instead.'

after I run mw.loader.load('mediawiki.toolbar') and click on the links again.

@Krinkle, it's worse, since the whole extension relies on parsing the wikicode of an interface message, and it's only reliable connection between 'desired functionality' and the extension delivering the functionality is that parserhook. This makes it rather difficult to properly correct this problem using RL (or actually even without using RL) until we enable merging i18n parser RL modules into the output page (didn't bartosz have a patch attempt for that somewhere?)

I don't see a good way to solve this quickly though...
Perhaps we should consider restoring the global insertTags() function temporarily.

In rowiki case enabling edit toolbar does not help. Better said, i already had enabled this feature and issue still occur either with or without feature enabled. Problem generally appear only on [[Special:Upload]] page - see He7der's post above.

Also on rowiki, recently one editor reported in local village pump a total disappearance of edit toolbar (https://ro.wikipedia.org/w/index.php?title=Wikipedia:Cafenea&oldid=9315395#Caseta_de_modificare), another one confirming periodical disappearance. I also noticed this issue. See T96043

@matmarex :

as T92781 was closed as duplicate of this task, I draw attention again that :ro:MediaWiki:Edittools does not work on :ro:Special:Upload.

While it works on action=edit depending on the user's personal configuration, it's always broken on [[Special:Upload]].

Change 288922 had a related patch set uploaded (by Brian Wolff):
[DO NOT MERGE] Remove inline event handler js from charinsert

https://gerrit.wikimedia.org/r/288922

Change 288922 merged by jenkins-bot:
Remove inline event handler js from charinsert

https://gerrit.wikimedia.org/r/288922