fix: thread disappearing after refresh

[GretaD]

fixes #9759

to test

1. open mail app on list view
2. open a message
3. refresh

[ChristophWurst]

Will this cause double fetching when you don't refresh the page but navigate from one mailbox to another?

[GretaD]

Will this cause double fetching when you don't refresh the page but navigate from one mailbox to another?

very good point/question. I will add a check to see if the envelopes are already being loaded before making the fetch request.