Buddypress: Expand autocomplete mentions list

bp-custom.php

//turn on @mentions for all users
define( 'BP_MESSAGES_AUTOCOMPLETE_ALL', true );

/*---- Expand Auto Complete -------*/
//https://github.com/buddypress/BuddyPress/blob/fe47890c1ab7e2cb24c005f1c35fb9c6c8c8ab7c/src/bp-core/classes/class-bp-members-suggestions.php#L85
function bp_filter_buddypress_auto_complete( $args ) {
    $args['per_page'] = 100;
   // $args['populate_extras'] = true;
    //$args['search_terms'] = 'some term';
    $args['search_wildcard'] = 'both';
    
    //possible exclude non-members
	
	return $args;
}
add_filter( 'bp_members_suggestions_query_args', 'bp_filter_buddypress_auto_complete' );

add above code to your bp-custom.php list

Also manually edit JS limit to 100:
https://github.com/buddypress/buddypress/blob/bbf1de1201e46ad2b05065185c7a54a9f9a4a858/src/bp-activity/js/mentions.js#L34

Buddypress: buddypress/src/bp-activity/js/mentions.js
Buddyboss: buddyboss-platform/bp-core/js/mentions.js

Add CSS to child theme style.css:
.atwho-view ul {
max-height: 300px;
overflow-y: scroll;
}