Fix scroll being stuck at bottom

[dbkr]

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 element-hq/element-web#18903

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix scroll being stuck at bottom (#6751). Fixes Scrolling impossible when there are unread messages out of screen element-hq/element-web#18903.

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