I had this for years on my old PC, a sort of display bug that I also managed to get rid of by changing the padding. But it doesn't come from XenForo, I don't know if it comes from your screen, from Windows or something else. Browse XF on another PC and you probably won't get the same thing... I changed PCs last month and it disappeared.
I have noticed a small border problem for some time:
There is a space, I guess 1px, to the left of all the blocks on my site. This space is located between the container block and its contents. I searched for hours and hours in my extra.less for what could create this problem but I did not find anything. At the end I even completely emptied extra.less but the problem persisted.
In the style properties I also went through my modifications but nothing allowed to delete this space. I have consulted other forums and on some I have observed the same phenomenon. Weird.