The row in a conversation that is not a message
Date breaks, joins, mode changes, system notes. A marker sits between two divider lines in quiet tertiary text, so the eye skips it while scanning turns and screen readers treat it as a boundary rather than a message.
Date breaks
The most common marker. Keep the label short: a day name, a date, nothing more.
System notes
An optional leading icon gives an event a glyph. The icon is decorative; the label carries the meaning.
Without lines
Turning the lines off keeps the label centred on the same geometry. Use it when markers stack close together and the rules would start to read as a table.
In a conversation
Markers punctuate the transcript without joining it. Events belong here; system messages with real content belong in a chat message.
Where did we land on the sidebar layout?
We kept the searchable variant and parked the density work.
Picking this back up. Can you summarise the open questions?