I’ve been fighting a weird and, honestly, low-priority bug for the last several releases. I haven’t seen it on the forums, but it’s present on both my work computer (which uses Chrome) and home computer (which uses Firefox), and across multiple installations.
Randomly, when editing a page, opening the HTML editor on any element that uses it will cause the container for the entire page preview to expand vertically by about 25px, and you can no longer scroll down to that section (because the bottom of the page is now outside of the viewport). Closing and re-opening the HTML editor will cause it to expand by another ~25px, meaning that editing a page for any length of time will cause it to subtly grow longer until all that you can “reach” is the top 1/3 of the page or so. The rich text and raw editors do not seem to be impacted, although switching between those and the HTML editor without closing the content editor also causes the page to expand.
I haven’t been able to figure out what triggers it–it seems to happen randomly regardless of what elements, configurations, loopers, or settings I have in place, but it does seem to affect most Pro installations. Saving my work and refreshing the page corrects the page preview height and temporarily fixes the bug.