I’m having this problem fairly consistently (using Firefox on my Linux laptop), but have discovered a work-around: (1) Start to type your reply, then (2) Click on the minimise down-arrow at the upper-right of the commenting area (above where the preview should be but isn’t). After the commenting area minimises, (3) Click on the restore up-arrow (being careful to not click on the destroy reply cross (X)!). Voilà, all(?) now works!