There's a part of bug 1551582 that's still happening even after that bug got fixed.
Steps to reproduce:
- Go to irccloud.mozilla.com and log in with your mozilla LDAP credentials.
- Go to a channel which has at least one or two pages of unread scrollback.
- Scroll to the very bottom. There should be a bar at the top that says "[some time span] of unread messages".
- Press the Esc key.
The bar at the top should disappear and the scrolled contents should remain in the same place.
The bar at the top disappears, but the scrolled contents jump up by the size of the bar. There is blank space at the bottom.
Scrolling resets this odd state back to the normal state; scrolling even very slightly makes the gap disappear instantly.
I'm assuming that this bug was caused by bug 1549625, just like bug 1551582 was, but I haven't verified this.