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 element-hq/element-web#18903

Author: dbkr

src/components/structures/ScrollPanel.tsx

@@ -275,8 +275,8 @@ export default class ScrollPanel extends React.Component<IProps> {
         // fractional values (both too big and too small)
         // for scrollTop happen on certain browsers/platforms
         // when scrolled all the way down. E.g. Chrome 72 on debian.
-        // so check difference <= 1;
-        return Math.abs(sn.scrollHeight - (sn.scrollTop + sn.clientHeight)) <= 1;
+        // so check difference < 1;
+        return Math.abs(sn.scrollHeight - (sn.scrollTop + sn.clientHeight)) < 1;
     };
 
     // returns the vertical height in the given direction that can be removed from