when useWindow is false,offset calculate wrong

[xu-snow]

In L152

offset =
        el.scrollHeight - el.parentNode.scrollTop - el.parentNode.clientHeight;

if el has brother node before it,the offset may calculated less than actuality,i think it should add el.offsetTop,like:

 offset =
        el.scrollHeight +el.offsetTop - el.parentNode.scrollTop - el.parentNode.clientHeight;

[danbovey]

I recently updated the issue templates on this repo so that I can identify the bugs with this repo.

Please clone your layout and use of react-infinite-scroller by forking this Code Sandbox and linking it here. Doing so will massively expedite getting the bug fixed! 👊