We need to reset the scroll offset otherwise the component may be
scrolled past the only content it has (Chrome just corrected the
scroll offset but Firefox scrolled it anyway).
NB. Introducing the new deriveStateFromProps method seems to
means that react no longer calls componentWillMount so I've
had to change it to componentDidMount (which it should have
been anyway).
Fixes https://github.com/vector-im/riot-web/issues/11263
David Baker
7d6643664e