Flow Part widget editor open/close buttons are broken

[Piedone]

Describe the bug

If you add a widget to Flow Part, the arrow open and close buttons are both displayed, are misaligned, and don't do anything on a click.

This is a 2.0 regression.

To Reproduce

1. Use latest main (125fd39).
2. Set up new site with the Blog recipe.
3. Create a Page.
4. Try to add any widget to Flow Part.
5. Observe the above-mentioned issue. No log entries or browser exceptions.

Expected behavior

We see the same as with 1.8.

Screenshots

For reference, here's how it looks like under 1.8: