Memory track hover dot affects scrollable size in header

Steps to reproduce:

1. Go to https://perfht.ml/2CpYC2S
1. Scroll down in the header to the very bottom.
1. Hover the memory track for Web Content (2/2) in the right half of the track, where the orange graph is very low.
1. Without moving the mouse, scroll down.
1. Unhover.

Expected results:
At step 4 you shouldn't be able to scroll down more.
So that step 5 doesn't make the header jerk.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-113)