When you change tabs in the Lower Zone and the Editor loses focus, it will forget which event(s) it was editing.
It acts as if the “window” was closed, but in a tab system you intuitively consider that changing tabs means you sending the current one to the background, not to close it.
With the full Editor window, if the window is sent to the background and then opened again then the content is still there. Instead you get the “No Editor Open” message, which serves no purpose.
Here is an example of this behaviour:
This does not happen with the full Key Editor: