From b.m.o Bug 1831787.

Requires overlay scrollbars to be on (All platforms except Windows 10).

STR:

1. On SuMo, select edit on any article
2. Change language to non-English, which brings up the translate dialog
3. Hover mouse over english/non-english boxes, checking for scrollbar

Expected:
Scrollbar becomes visible for both boxes, indicating more content

Actual:
No scrollbar visible on the right side box until trying to scroll it

CodeMirror widget seems to manage its own scrollbar.