When you change tabs in the Lower Zone and the event you’re editing isn’t selected anymore, it will forget which event(s) it was editing. This means you can’t do any project operations that causes deselection, unless you never change tabs.
Changing tabs behaves as if the tab was closed instead of sent to the background. However, a windowed editor would remember the last selected event(s) when sent to the background, which is more useful than a “No Editor Open” message.
I turned this into a feature request instead of an Issue report on the recommendation of Martin.Jirsak