I was just about to figure out how to file a bug report about this issue! Only happens when using the Dark Thunderbird theme for me using 80.0b5 on Ubuntu 18.04.5 LTS. If I switch to the Default or Light theme the summaries appear in the Message pane with the threads collapsed. That is after I restart so the missing items from the Folder and Thread panes reappear.
Bug 1660403 Comment 5 Edit History
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I was just about to figure out how to file a bug report about this issue! Only happens when using the Dark Thunderbird theme for me using 80.0b5 on Ubuntu 18.04.5 LTS. If I switch to the Default or Light theme the summaries appear in the Message pane with the threads collapsed. That is after I restart so the missing items from the Folder and Thread panes reappear. The summary properly appears in the Message pane using the TB Dark theme in my 80.0b5 on Windows10.