I'm not sure how to cause or remedy this – this morning I switched to the list view in the calendar sidebar. It showed my calendar items fine. Later it showed nothing, perhaps after the machine returned from sleep. [EDIT – sleep is not a factor, but time passing is. I saw sidebar contents, joined a Zoom meeting for an hour, and the time I left the meeting the sidebar was empty again.] The timeline view is still correct. The list view will repopulate if I change days.