Force notebook scrollbar update after content change

[msujew]

What it does

Forces the scrollbar that hosts the notebook widget to update after the content has changed.

This is relevant when the user has used the Clear All Outputs command while the widget is scrolled to the end of the content. In this case the scrollbar was effectively after the content. Updating the scrollbar ensures that the scrollbar doesn't allow scrolling outside of the content.

How to test

1. Create a notebook document that has large outputs
2. Scroll to the end of the widget
3. Run the Clear All Output command
4. Ensure that the document is still visible

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[jonah-iden]

Works great 👍