Optimize display of large text bodies

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

It turns out calculating the width of a text body is expensive for large bodies, because we have to count every grapheme. This change caches the text dimensions so we only have to calculate it when the text changes.

Further progress on #356

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

Code complexity

QA

How did you test this?

manually. No functionality change means it's hard to write tests for this

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked