The "clock"/loading icon is from here:
https://github.com/kovidgoyal/calibr...or/__init__.py
I can understand why you'd expect it to indicate a "finished" state as it does in the main GUI.
But this still isn't the right subforum.
The sticky clearly states that the Development subforum is ONLY for asking questions about how to use or modify the calibre source code.
As a special exception, if your question is about fundamental design decisions, you will be kicked out* of the Development forum no matter what -- this is because certain people kept trying to "fix the code" in ways Kovid specifically refused to accept.

This does not imply that minor design decisions are acceptable -- they are still only acceptable if the question involves YOU writing the code.
Your question is a bug report, about the editor, and as such belongs in the Editor subforum.
* -- post moved