Steel code view widget is slow when loading code for a complex `Graph`

[mitchmindtree]

This only happens when switching between graphs with the widget open, or when opening the widget for the first time with a complex graph selected.

I'm guessing this has something to do with running the text layout on a lot of code.

Ideally we can speed this up directly somehow so we can continue to load the widget synchronously, otherwise we might want to consider loading the text asynchronously (showing some placeholder spinner in the meantime) to avoid lagging the main thread.