Move threads e2e tests over to cypress (#8501)

* Add non-consent (default) Synapse template

* Add consent test

* Add create room test

* Stash work

* Initial threads tests

* fix

* Delete old threads e2e tests, plan new ones

* Fix typed s'more

* Try something else

* specify d.ts

* Fix types once and for all?

* Fix the consent tests

* Iterate threads test harness

* Fix dispatcher types

* Iterate threads test

* fix typing

* Alternative import attempt

* let it break let it break let it break

* Tweak types

* Stash

* delint and update docs

* null-guard scrollIntoView

* Iterate threads test

* Apply suggestions from code review

src/components/structures/TimelinePanel.tsx

@@ -1197,6 +1197,7 @@ class TimelinePanel extends React.Component<IProps, IState> {
 
     private scrollIntoView(eventId?: string, pixelOffset?: number, offsetBase?: number): void {
         const doScroll = () => {
+            if (!this.messagePanel.current) return;
             if (eventId) {
                 debuglog("TimelinePanel scrolling to eventId " + eventId +
                     " at position " + (offsetBase * 100) + "% + " + pixelOffset);