Toolbox animation broken

[srietkerk]

We had code changes a couple years ago to add animation to the toolbox when entering the Monaco editor. It seems like this no longer works.

For repro:

1. Go to Microbit
2. Create a project
3. Start in blocks and switch to JS
4. No animation on the toolbox

PR that made the changes awhile back: https://github.com/microsoft/pxt/pull/6774