Fix mark as unread button (#3393)

* Fix mark as unread button

* Revert to prefer the last event from the main timeline

* Refactor room test

* Fix type

* Improve docs

* Insert events to the end of the timeline

* Improve test doc

spec/test-utils/thread.ts

@@ -115,6 +115,26 @@ type MakeThreadProps = {
     ts?: number;
 };
 
+type MakeThreadResult = {
+    /**
+     * Thread model
+     */
+    thread: Thread;
+    /**
+     * Thread root event
+     */
+    rootEvent: MatrixEvent;
+    /**
+     * Events added to the thread
+     */
+    events: MatrixEvent[];
+};
+
+/**
+ * Starts a new thread in a room by creating a message as thread root.
+ * Also creates a Thread model and adds it to the room.
+ * Does not insert the messages into a timeline.
+ */
 export const mkThread = ({
     room,
     client,
@@ -122,7 +142,7 @@ export const mkThread = ({
     participantUserIds,
     length = 2,
     ts = 1,
-}: MakeThreadProps): { thread: Thread; rootEvent: MatrixEvent; events: MatrixEvent[] } => {
+}: MakeThreadProps): MakeThreadResult => {
     const { rootEvent, events } = makeThreadEvents({
         roomId: room.roomId,
         authorId,