Messages marked as unread in threads should stay unread until thread is opened

[fwille]

Is your feature request related to a problem? Please describe.
I can mark messages in threads as unread, but they lose this marking as soon as I navigate to the channel.
Currently this feature is more like a "mark channel as unread" as it isn't tied to the particular message. The option to mark a particular message as unread is therefore misleading.

Describe the solution you'd like
Messages marked as unread in threads should stay unread until the particular thread they are in is opened again. So I can still read other messages in the channel and get back to the particular thread message later.