Redmine

Currently, when a user without permission to view a particular issue is mentioned, they do not receive a notification email. However, visually, the mention link appears identical to others, which can be misleading as there's no indication that no email was sent.

In issue Feature #39528: Change mention link color for users without access to view issues, I proposed changing the background color of mention links for such users to make it clear they are non-notifiable. However, I believe that visual change alone may not sufficiently convey the intent to users. Therefore, I suggest using tooltips to display a message clarifying the status of the mention.
Yet, since tooltips are only used for displaying title attributes right now, expanding their use might require some discussion, which is why I have created a separate ticket for this matter.