The four React applications and the packages they share.
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The lesson editor is where an instructor writes a lesson: a rich text editor for the body, a place for a video link, file attachments, and a preview showing what a student will see. Instructors spend hours in it. It is, more than anything else we ship, the thing our customers actually work in.
And for a few months, it had been getting worse.
A bulk-enrolment dialog stopped opening. No error, no spinner, nothing — the button simply did not respond. Everything around it worked. It worked in development. It worked against a local preview of the production build. It worked on a colleague's machine.
Then we shipped a fix for something unrelated, and the dialog came back on its own. The search and filter bar stopped working instead.
A learner scored 120 out of 300 on a project. The admin console showed 300%. The instructor console showed 0%. The student's own course card showed 12000%.
Nobody had written a bug. Every one of those numbers came out of code that was individually reasonable.