Open two tabs to this glitch.
In one tab, click the first checkbox. It starts writing to the books db and keeps transactions open for a while. This simulates a client that has some long-running transactions it can't explicitly commit, which potentially block other transactions.
Switch to tab the second and click "Start reading library". Note the number of books read per second by this tab while it's in the foreground and the other tab in the background is doing lots of work.
With--enable-features=IdbPrioritizeForegroundClientsyou will notice that the bg tab doesn't interfere with the foreground tab.