Debounce read marker update on scroll

Reverts https://github.com/matrix-org/matrix-react-sdk/pull/6751 in
favour of debouncing the updates to read markers, because it seems
allowing the scroll to be 1px away from the bottom was important for
some browsers and meant they never got to the bottom.

We can fix the problem instead by debouncing the update to read
markers, because the scroll state gets reset back to the bottom when
componentDidUpdate() runs which happens after the read marker code
does a setState(). This should probably be debounced anyway since
it doesn't need to be run that frequently.

Fixes https://github.com/vector-im/element-web/issues/18961
Type: bugfix

src/components/structures/ScrollPanel.tsx

@@ -276,7 +276,7 @@ export default class ScrollPanel extends React.Component<IProps> {
         // 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;
+        return Math.abs(sn.scrollHeight - (sn.scrollTop + sn.clientHeight)) <= 1;
     };
 
     // returns the vertical height in the given direction that can be removed from

src/components/structures/TimelinePanel.tsx

@@ -47,11 +47,14 @@ import { RoomPermalinkCreator } from "../../utils/permalinks/Permalinks";
 import Spinner from "../views/elements/Spinner";
 import EditorStateTransfer from '../../utils/EditorStateTransfer';
 import ErrorDialog from '../views/dialogs/ErrorDialog';
+import { debounce } from 'lodash';
 
 const PAGINATE_SIZE = 20;
 const INITIAL_SIZE = 20;
 const READ_RECEIPT_INTERVAL_MS = 500;
 
+const READ_MARKER_DEBOUNCE_MS = 100;
+
 const DEBUG = false;
 
 let debuglog = function(...s: any[]) {};
@@ -475,6 +478,20 @@ class TimelinePanel extends React.Component<IProps, IState> {
         }
 
         if (this.props.manageReadMarkers) {
+            this.doManageReadMarkers();
+        }
+    };
+
+    /*
+     * Debounced function to manage read markers because we don't need to
+     * do this on every tiny scroll update. It also sets state which causes
+     * a component update, which can in turn reset the scroll position, so
+     * it's important we allow the browser to scroll a bit before running this
+     * (hence trailing edge only and debounce rather than throttle because
+     * we really only need to update this once the user has finished scrolling,
+     * not periodically while they scroll).
+     */
+    private doManageReadMarkers = debounce(() => {
         const rmPosition = this.getReadMarkerPosition();
         // we hide the read marker when it first comes onto the screen, but if
         // it goes back off the top of the screen (presumably because the user
@@ -488,8 +505,7 @@ class TimelinePanel extends React.Component<IProps, IState> {
         const timeout = this.readMarkerTimeout(rmPosition);
         // NO-OP when timeout already has set to the given value
         this.readMarkerActivityTimer.changeTimeout(timeout);
-        }
+    }, READ_MARKER_DEBOUNCE_MS, { leading: false, trailing: true });
-    };
 
     private onAction = (payload: ActionPayload): void => {
         switch (payload.action) {