Why does a detached tmux session now pause after modest output?

[Jim Lloyd]

For a few years now I have been routinely keeping a tmux session running on Linux workstation (Ubuntu 20). I often have run a simulation application that runs for 12 hours or more, and outputs a status line every 30s. I detach the session and return later, expecting the session to have continued running. But recently these sessions are paused after outputting what I think is a modest number of status lines (~500). I haven't been able to find a definitive answer on what setting is causing this.

I initially assumed that iTerm would not be responsible for this problem and Istill would not be surprised if iTerm is not responsible, but I have to wonder if a recent change to iTerm may have somehow caused the problem,  by changing in some way how it interacts with tmux sessions. This is because to my knowledge I have not changed. anything with my tmux installation on the server, but I do routinely allow iTerm updates.

If the problem is purely server side, can anyone tell me what I need to change?

If this is simply a buffer limit, I'd like to bump the limit up to 10K lines or more. I have lots of RAM on this server do am not concerned about the buffer filling memory.

Thanks

Jim