Don't consider threads for breaking continuation until actually created (#8581)

* Don't consider threads for breaking continuation until they've actually been created

* Update tests

* Make hasThreadSummary null thread safe

* Apply feedback from pr review

src/components/structures/MessagePanel.tsx

@@ -56,6 +56,7 @@ import { getEventDisplayInfo } from "../../utils/EventRenderingUtils";
 import { IReadReceiptInfo } from "../views/rooms/ReadReceiptMarker";
 import { haveRendererForEvent } from "../../events/EventTileFactory";
 import { editorRoomKey } from "../../Editing";
+import { hasThreadSummary } from "../../utils/EventUtils";
 
 const CONTINUATION_MAX_INTERVAL = 5 * 60 * 1000; // 5 minutes
 const continuedTypes = [EventType.Sticker, EventType.RoomMessage];
@@ -96,7 +97,7 @@ export function shouldFormContinuation(
 
     // Thread summaries in the main timeline should break up a continuation on both sides
     if (threadsEnabled &&
-        (mxEvent.isThreadRoot || prevEvent.isThreadRoot) &&
+        (hasThreadSummary(mxEvent) || hasThreadSummary(prevEvent)) &&
         timelineRenderingType !== TimelineRenderingType.Thread
     ) {
         return false;

src/utils/EventUtils.ts

@@ -280,3 +280,7 @@ export function canForward(event: MatrixEvent): boolean {
         M_POLL_START.matches(event.getType())
     );
 }
+
+export function hasThreadSummary(event: MatrixEvent): boolean {
+    return event.isThreadRoot && event.getThread()?.length && !!event.getThread().replyToEvent;
+}

test/components/structures/MessagePanel-test.js

@@ -699,7 +699,7 @@ describe('MessagePanel', function() {
 });
 
 describe("shouldFormContinuation", () => {
-    it("does not form continuations from thread roots", () => {
+    it("does not form continuations from thread roots which have summaries", () => {
         const message1 = TestUtilsMatrix.mkMessage({
             event: true,
             room: "!room:id",
@@ -730,6 +730,14 @@ describe("shouldFormContinuation", () => {
         });
 
         expect(shouldFormContinuation(message1, message2, false, true)).toEqual(true);
+        expect(shouldFormContinuation(message2, threadRoot, false, true)).toEqual(true);
+        expect(shouldFormContinuation(threadRoot, message3, false, true)).toEqual(true);
+
+        const thread = {
+            length: 1,
+            replyToEvent: {},
+        };
+        jest.spyOn(threadRoot, "getThread").mockReturnValue(thread);
         expect(shouldFormContinuation(message2, threadRoot, false, true)).toEqual(false);
         expect(shouldFormContinuation(threadRoot, message3, false, true)).toEqual(false);
     });