The way Diffy takes screenshots is we open a page in a browser with a really large height (10.000+ pixels). Usually it works just fine, but if site has some elements that has CSS height property set in % or vh units these elements will get stretched.
We have came up with a workaround for this -- you can override CSS of your site prior to taking the screenshot.
For that simply find the elements that are stretched and use the same selectors to set their height in pixels.
So then you can use a CSS rule to fix the height of the block.