Don't form continuations on either side of a thread root (#8408)

Author: t3chguy

src/components/structures/MessagePanel.tsx

@@ -92,9 +92,13 @@ export function shouldFormContinuation(
         mxEvent.sender.name !== prevEvent.sender.name ||
         mxEvent.sender.getMxcAvatarUrl() !== prevEvent.sender.getMxcAvatarUrl()) return false;
 
-    // Thread summaries in the main timeline should break up a continuation
-    if (threadsEnabled && prevEvent.isThreadRoot &&
-        timelineRenderingType !== TimelineRenderingType.Thread) return false;
+    // Thread summaries in the main timeline should break up a continuation on both sides
+    if (threadsEnabled &&
+        (mxEvent.isThreadRoot || prevEvent.isThreadRoot) &&
+        timelineRenderingType !== TimelineRenderingType.Thread
+    ) {
+        return false;
+    }
 
     // if we don't have tile for previous event then it was shown by showHiddenEvents and has no SenderProfile
     if (!haveRendererForEvent(prevEvent, showHiddenEvents)) return false;

test/components/structures/MessagePanel-test.js

@@ -674,6 +674,20 @@ describe('MessagePanel', function() {
 
 describe("shouldFormContinuation", () => {
     it("does not form continuations from thread roots", () => {
+        const message1 = TestUtilsMatrix.mkMessage({
+            event: true,
+            room: "!room:id",
+            user: "@user:id",
+            msg: "Here is a message in the main timeline",
+        });
+
+        const message2 = TestUtilsMatrix.mkMessage({
+            event: true,
+            room: "!room:id",
+            user: "@user:id",
+            msg: "And here's another message in the main timeline",
+        });
+
         const threadRoot = TestUtilsMatrix.mkMessage({
             event: true,
             room: "!room:id",
@@ -682,14 +696,15 @@ describe("shouldFormContinuation", () => {
         });
         jest.spyOn(threadRoot, "isThreadRoot", "get").mockReturnValue(true);
 
-        const message = TestUtilsMatrix.mkMessage({
+        const message3 = TestUtilsMatrix.mkMessage({
             event: true,
             room: "!room:id",
             user: "@user:id",
-            msg: "And here's another message in the main timeline",
+            msg: "And here's another message in the main timeline after the thread root",
         });
 
-        expect(shouldFormContinuation(threadRoot, message, false, true)).toEqual(false);
-        expect(shouldFormContinuation(message, threadRoot, false, true)).toEqual(true);
+        expect(shouldFormContinuation(message1, message2, false, true)).toEqual(true);
+        expect(shouldFormContinuation(message2, threadRoot, false, true)).toEqual(false);
+        expect(shouldFormContinuation(threadRoot, message3, false, true)).toEqual(false);
     });
 });