Fix scroll being stuck at bottom
The check for whether we're at the bottom or not allowed for a
difference of 1 to account for fractional scroll values, but
allowed the difference of exactly 1 too, meaning we'd consider
the timeline to be at the bottom if you were scrolled up by exactly
a single pixel. If your scrolling was set up to be precise enough and
the event handlers fired fast enough that they'd evaluate each time
you scrolled up by a single pixel, it would reset you back to the bottom
each time and you'd never be able to scroll up.
Fixes https://github.com/vector-im/element-web/issues/18903
