[Toolkits] CodeMirrror seems to fade not collapse unlike other content

[microbit-matt-hillsdon]

If there's a CodeMirror displaying a code snippet in the toolkit then when you "Show less" then it seems to fade out but not slide up.

See e.g. display.scroll.

Clearer if you remove the other content in the Collapse from the DOM.

Perhaps related to #301.

[microbit-matt-hillsdon]

This was fixed as part of content visibility / animation changes in Reference.