Setting explicit rights to particular user denies all existing group rights

1. As Admin:
   1. Create group 'TestGroup'
   2. Create Users Test1 and Test2
   3. Add users to 'TestGroup'
   4. Grant privilege for 'TestGroup' to 'view' XWiki space (on the Group rights edit page)
   5. Install Tooltip Macro
2. As test user: *Verify that macro is invisible* to test users by visiting macro page http://localhost/xwiki/bin/view/XWiki/Tooltip
3. As Admin:
   1. Go to users rights edit of the XWiki.Tooltip
   2. Uncheck 'view' checkbock for unregistered users (see the screenshot)
   3. Also verify that for all other users all check boxes are empty
4. As test user verify that access to the tooltip page has been restored

You can repeat the above steps just substituting 'Unregistered users' with 'Test2' to see permissions disappearing for 'Test1'

Even though it may be treated as feature, I rather think that it's a bug, because it's counter-intuitive (and frustrating to debug) that a group looses access it some per-user right has been applied. There's also a use case where user rights may need to be applied with group rights - when you want to show particular page to the user that does not belong to the group with space visibility.