Generate Press and WordPress problems

How the masonry grid looks in WP’s Customizer (in Firefox) – above the fold it looks fine, but below there are unexplained spaces
This is the same page as it appears in Firefox not accessed through the WP Dashboard. There are more unexplained space, but in different places
This is the same page accessed in Opera – fewer gaps but they are still there
And here is the same page again in Chrome. Not only are there gaps, but there’s some sort of weird overwriting going on as well