Every time the users switch between tabs in Milestone and to out application, they are required to log in again.
If you have a ViewItem plugin it is closed when you switch to another tab, and is reopened when you switch back. This is base functionality that cannot be changed. You will need to figure a way to persist your login somehow.
PS. You cannot reference information from a non public site in this public developer forum. If my answer does not fit you will have to mention information that is otherwise missing.
I have been thinking; one way could be to have functionality running in a background plugin and then use it in your view item plugin. I guess there are many ways so if you get stuck you can ask further questions here.
Thank you for your response, we will look into this suggestion.