If you use 'Threads' and a thread is in a 'collapsed' state then clicking on that collapsed threads means open all threads therein in separate tabs - this has always been the means of opening up everything because you are clicking on an entire thread and not an individual mail in thread.

If you 'expand' the thread first then you can click on any email in that thread to display in either Mesasge Pane or in separate tab. It will only open the selected email.

The alternative to see all comments in a thread is to use a right click and select 'Open' > 'Open Message in conversion' and that opens one tab with all other threads listed both incoming and outgoing.

Thank you, that is super helpful