Thanks for that. It looks like you need to start troubleshooting your plugins and theme components. Try /safe-mode and turn off unofficial plugins to start with.
Sorry you’re having this problem! It can be frustrating to have an issue that can’t be replicated on other sites and pinned down easily.
A long shot, but can you take a scan through the latest staff action logs at /admin/logs/staff_action_logs to see if a site setting was changed recently that might affect this?
We’re currently configuring our Discourse as part of an upgrade from 2.4 so our logs are a chaotic mess at the mo - and this issue has only just been spotted, so it’s not going to be possible to identify the possible cause in the logs (the majority of which is installing/configuring Themes/Theme Components rather than site settings).
Found the cause… it was a message that had lots of groups/roles in the ‘to’ field, as the groups are displayed in one long line under the subject line it pushed the inbox area wider than the UX, archiving the message made screen look normal.
This is a possible bug, messages with a long lists of groups in the ‘To’ pushes the width of the inbox screen. Maybe the groups should go on multiple lines?