Adds a small loading indicator for notebook widgets. While the notebook is loading (which might include activating the corresponding extension and waiting until the notebook serializer is ready), we simply display the indicator to make it clear that the notebook is still loading.
How to test
Open a notebook
Assert that for a short duration, the indicator is visible at the top of the widget.
Review checklist
[x] As an author, I have thoroughly tested my changes and carefully followed the review guidelines
What it does
Adds a small loading indicator for notebook widgets. While the notebook is loading (which might include activating the corresponding extension and waiting until the notebook serializer is ready), we simply display the indicator to make it clear that the notebook is still loading.
How to test
Review checklist
Reminder for reviewers