Summary:
There was a bug where stale Markdown values were sometimes shown when moving between pages. It appears to have been an issue with async rendering of Markdown and how cell values were being updated.
As a fix, we now use synchronous rendering, which doesn't have any perceptible performance penalty, and behaves predictably.
Test Plan: Manual.
Reviewers: jarek
Reviewed By: jarek
Subscribers: jarek
Differential Revision: https://phab.getgrist.com/D4379