I found a solution for this problem. Disabling Refresh interval by set 0, it stopped jump in my case.
But without Refresh interval there are different size of empty space to bottom on different post pages. It depends on size of post.
So, I can not say that above action is the fix of the problem.
I have the same problem. Setting Refresh interval to 0, sorted out the jumping, but now sometimes the fixed widget stays on the page until the very end, other times it disappears towards the end. Really inconsistent. I hope this gets sorted out soon.