This test demonstrates your browser's treatment of concurrent IndexedDB readwrite transactions. If you see a spinner rotating, it's using system resources, so you probably don't want to keep this tab open for too long in that state.
For browsers which impose the bare minimum set of constraints on transaction scheduling (see spec), this test should complete very quickly.
For browsers that apply additional constraints, in particular disallowing concurrent readwrite transactions on the same IDB database, this test will never complete. Technically either behavior is standards-compliant and IRL sites are (hopefully) unlikely to behave as this one does. However, this behavior also indicates a possible performance bottleneck for real world sites that concurrently read and write to multiple object stores in the same database, and/or do extensive work in transactions.
One take-away for site owners is to prefer using distinct databases rather than simply separate object stores