From b3e6cbfddd7115bfd9aa1d44fd6535bba3ec2883 Mon Sep 17 00:00:00 2001
From: Bruno Windels <brunow@matrix.org>
Date: Thu, 21 Feb 2019 18:51:18 +0100
Subject: [PATCH] restore scroll state when timeline resizes using
 ResizeObserver

(only where supported, polyfill doesn't give good results)
---
 src/components/structures/ScrollPanel.js | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/components/structures/ScrollPanel.js b/src/components/structures/ScrollPanel.js
index a1a7d08e0b..db19b1d0cc 100644
--- a/src/components/structures/ScrollPanel.js
+++ b/src/components/structures/ScrollPanel.js
@@ -160,6 +160,13 @@ module.exports = React.createClass({
 
     componentDidMount: function() {
         this.checkScroll();
+
+        if (typeof ResizeObserver !== "undefined") {
+            this._timelineSizeObserver = new ResizeObserver(() => {
+                this._restoreSavedScrollState();
+            });
+            this._timelineSizeObserver.observe(this.refs.itemlist);
+        }
     },
 
     componentDidUpdate: function() {
@@ -181,6 +188,10 @@ module.exports = React.createClass({
         //
         // (We could use isMounted(), but facebook have deprecated that.)
         this.unmounted = true;
+        if (this._timelineSizeObserver) {
+            this._timelineSizeObserver.disconnect();
+            this._timelineSizeObserver = null;
+        }
     },
 
     onScroll: function(ev) {