Only send threaded read receipts if threads support is enabled (#3612)

* Only send threaded read receipts if threads support is enabled

* Tests

spec/unit/read-receipt.spec.ts

@@ -55,6 +55,7 @@ describe("Read receipt", () => {
             fetchFn: httpBackend.fetchFn as typeof global.fetch,
         });
         client.isGuest = () => false;
+        client.supportsThreads = () => true;
 
         threadEvent = utils.mkEvent({
             event: true,
@@ -178,6 +179,29 @@ describe("Read receipt", () => {
             await httpBackend.flushAllExpected();
             await flushPromises();
         });
+
+        it("should always send unthreaded receipts if threads support is disabled", async () => {
+            client.supportsThreads = () => false;
+
+            httpBackend
+                .when(
+                    "POST",
+                    encodeUri("/rooms/$roomId/receipt/$receiptType/$eventId", {
+                        $roomId: ROOM_ID,
+                        $receiptType: ReceiptType.Read,
+                        $eventId: roomEvent.getId()!,
+                    }),
+                )
+                .check((request) => {
+                    expect(request.data.thread_id).toEqual(undefined);
+                })
+                .respond(200, {});
+
+            client.sendReceipt(roomEvent, ReceiptType.Read, {});
+
+            await httpBackend.flushAllExpected();
+            await flushPromises();
+        });
     });
 
     describe("synthesizeReceipt", () => {

src/client.ts

@@ -4999,7 +4999,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             $eventId: event.getId()!,
         });
 
-        if (!unthreaded) {
+        if (!unthreaded && this.supportsThreads()) {
             // XXX: the spec currently says a threaded read receipt can be sent for the root of a thread,
             // but in practice this isn't possible and the spec needs updating.
             const isThread =