[minor] Seeing room top, expand stuff, can't scroll up

[grinapo]

Description

After room upgrade (and possibly any case where the upscrolling is limited) expanding anything (like the joins of the upgraded room) makes it impossible to scroll up, when looking at the top.

Looks like the scroll limit doesn't add the expanded size to the distance.

Steps to reproduce

• Go top of an upgraded room
• expand joins
• try to scroll to the top and see you cannot

But then:

• scroll down a few screens
• scroll upand it may work.

Version information

• Platform: web

For the web app:

• Browser: Chromium Version 74.0.3729.108 (Developer Build) built on Debian 10.0, running on Debian buster/sid (64-bit)
• OS: Debian/L
• URL: riot-web version: 1.2.2