Bigger scroll trigger when scrolling up in the comments

[icoloma]

When scrolling the comments view, it's not uncommon to scroll up a couple of lines to read something that you may have passed by skimming too fast. When you do that, the header will appear scrolling down and hiding the information that I wanted to see, forcing to scroll up even more. With a long post, this can happen multiple times.

Adding a bigger amount of scroll up to trigger the display of the header would solve this. This could get tricky with the initial page of comments.

This is not an important issue, and could just be ignored if the implementation is not trivial.