failing to persist initial sync events and fragments

[bwindels]

failing to persist initial sync on firefox with local "main" synapse on bruno2

I get a ton of:

Error: add on undefined failed: (name: TransactionInactiveError) (code: TransactionInactiveError) A request was placed against a transaction which is currently not active, or which is finished.

and no items in timelineEvents or timelineFragments are stored, presumably something is going wrong in the SyncWriter.
The error was not introduced in bwindels/fixconstrainterror-part2, and is already on master. Use git bisect to find out where it was introduced?

it's easy to repro on my local ff, just clear the bruno2 session and click on it to do an initial sync. the console is full of errors