## Summary of the Pull Request
Based on the discussion in #5479, it seems that the crash is caused by a race condition due to not obtaining the write lock before calling `TriggerFontChange`.
I'm not totally sure if my approach is the right one, but I've taken the lock out of `_RefreshSize` since it seems like all calls of `_RefreshSize` come after a `TriggerFontChange`/`UpdateFont`. Then I just
made sure all calls of `TriggerFontChange`/`UpdateFont` are preceded with a `LockForWriting`.
## PR Checklist
* [x] Closes#5479
* [x] CLA signed.
* [x] Tests added/passed
## Validation Steps Performed
Scrolling to change my font size does not kill the Terminal anymore! 🙌