Page elements getting gradually narrower down the page?

I've got a very strange issue. I'm trying to create a large page with hundreds of code blocks, and for some reason, the code blocks are narrowing as the page scrolls down.

Eventually, the page gets so narrow that the content becomes unreadable.

I am using site-wide custom CSS as well as page-specific header injection, which I thought could be the culprit, but I tried removing both and the problem persists.

Weirdly, when the site is in Edit Mode, everything displays exactly as it should.

I know Squarespace advises against using more than 60 blocks in a page to avoid loading issues, but I thought I'd proceed anyway since I'm not particularly concerned about load times. That said, perhaps the number of blocks is somehow creating the issue?

I'd be infinitely grateful for any insight or help, as I'm a novice web designer at best - and even that's pretty generous. ;)

Thanks for reading!