Steps to reproduce:
- Login with Admin
- Create a new global user named "globalUser"
- Create a new subwiki: select "Only global users are available in the wiki" as the User Scope and add globalUser as a member
- You'll get a merge conflict for XWiki.XWikiAllGroup: keep the current version of the document and click Continue
- Edit Main.WebHome inline and add the Message Sender gadget
- Select User from the drop-down list and begin typing "globalUser"
The search suggest will display "User not found".
I have tested the same scenarios on the main wiki and a subwiki with local users and I couldn't reproduce this issue.
Re XWIKI-6668: Also if you create a new group in the subwiki, add globalUser as a member and then send a message to that group, globalUser won't receive the message.