I do notice, FWIW, that sometimes slow page-loading causes the whole expanded Jetpack Comments form to be displayed. Perhaps the whitespace is related to this somehow?
I get the same impression. If I reload the page, it doesn’t happen again, which would make sense, because by that point I have all of the resources cached, so there is no reason for the site to load slowly. However, I do see it happening on other posts sometimes; again, it doesn’t repeat if I reload the page. I tried running your site through Pingdom’s tool, and the result wasn’t great:
https://tools.pingdom.com/fpt/#!/dpx8A5/https://jack-foreigner.com/chinese-suspense-novels/
It took a while for your server to respond, and there were several assets that were a 404. You might want to look into fixing those issues, and that will likely solve the white space issue with the comments form.
any way to change the default “Enter your comment here…” textfield value??
No, since the comments form is loaded from WordPress.com’s servers; we’d have to change the text there (and thus, for all WordPress.com sites) if we were to change the placeholder text.