Scrollbar not updated on tab switch

[ygoe]

Description

The scrollbar is not updated after switching the tab, until hovering the scrollbar with the mouse cursor.

Steps to Reproduce

1. Open this page: https://shellyparts.de/collections/hutschienenhalter (the URL is probably irrelevant)
2. Open this page in a second tab: https://github.com/dotnet/designs/blob/main/accepted/2020/form-factors.md#native-aot-form-factors
3. Switch to the first tab
4. Switch to the second tab

Actual result:

The first tab's scrollbar is displayed. Even if you scrolled to different offsets in both tabs, that part of the UI won't update to the second tab. Only when you hover the scrollbar with the mouse cursor, it's repainted.

Expected result:

Scrollbar repaints correctly every time the tab is switched.

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.33.106 Chromium: 96.0.4664.110 (Offizieller Build) (64-Bit)
Überarbeitung	d5ef0e8214bc14c9b5bbf69a1515e431394c62a6-refs/branch-heads/4664@{#1283}
Betriebssystem	Windows 10 Version 20H2 (Build 19042.1348)

[ygoe]

Additional test case: After switching to the second tab, minimise the browser window and then restore it. Sometimes you can see pixel garbage in the area of the scrollbar. But sometimes it doesn't reproduce. Try a few times. Most of the times it can be seen.