Scrolling on mobile causes menu mis-alignment

## Bug Report

**Current Behavior**
When scrolling with the menu open on mobile (confirmed on iOS with safari) the menu jumps around and appears on the right of the screen.
video : https://streamable.com/7ky6bo

**Steps to Reproduce**

As an admin, going to any profile, tapping triple dots in the user card shows the "dropdown" menu, when scrolling the issue then appears.
Can replicate on any iPhone, works fine on mobile device emulator on my PC.

**Environment**
- Flarum version: discuss and nightly
- Website URL: http://discuss.Flarum.org
- Browser: iOS 14.4 // Safari

**Confirmed by:**
- Katos
- @deebug

Start by reproducing the issue on an iPhone with iOS 14.4 and Safari: open an admin profile, tap the triple-dots menu in the user card, and scroll. Compare the behavior with the mobile emulator; done means the open menu stays correctly aligned while scrolling on mobile Safari.

Written by the indexing model from the issue text.