This is a weird grey area of the spec. We have a test for it here:
https://cs.chromium.org/chromium/src/third_party/blink/web_tests/storage/indexeddb/resources/transaction-starvation.js

This conflicts with how Scopes needs to lock things, and doesn't match what Firefox does (it fails this test).

We should change the spec to be explicit in this case - readwrite transactions cannot execute when a readonly transaction is executing.

It would also be nice to add a webplatformtest