diff --git a/src/logging/NullLogger.js b/src/logging/NullLogger.js
index da04f16b..060212bd 100644
--- a/src/logging/NullLogger.js
+++ b/src/logging/NullLogger.js
@@ -75,6 +75,8 @@ export class NullLogItem {
 
     refDetached() {}
 
+    ensureRefId() {}
+
     get level() {
         return LogLevel;
     }
diff --git a/src/matrix/room/timeline/persistence/GapWriter.js b/src/matrix/room/timeline/persistence/GapWriter.js
index ec23db5a..d51f4a18 100644
--- a/src/matrix/room/timeline/persistence/GapWriter.js
+++ b/src/matrix/room/timeline/persistence/GapWriter.js
@@ -253,3 +253,176 @@ export class GapWriter {
         return {entries, updatedEntries, fragments};
     }
 }
+
+import {FragmentIdComparer} from "../FragmentIdComparer.js";
+import {RelationWriter} from "./RelationWriter.js";
+import {createMockStorage} from "../../../../mocks/Storage.js";
+import {FragmentBoundaryEntry} from "../entries/FragmentBoundaryEntry.js";
+import {NullLogItem} from "../../../../logging/NullLogger.js";
+import {TimelineMock, eventIds, eventId} from "../../../../mocks/TimelineMock.ts";
+import {SyncWriter} from "./SyncWriter.js";
+import {MemberWriter} from "./MemberWriter.js";
+import {KeyLimits} from "../../../storage/common";
+
+export function tests() {
+    const roomId = "!room:hs.tdl";
+    const alice = "alice@hs.tdl";
+    const logger = new NullLogItem();
+
+    async function createGapFillTxn(storage) {
+        return storage.readWriteTxn([
+            storage.storeNames.roomMembers,
+            storage.storeNames.pendingEvents,
+            storage.storeNames.timelineEvents,
+            storage.storeNames.timelineRelations,
+            storage.storeNames.timelineFragments,
+        ]);
+    }
+
+    async function setup() {
+        const storage = await createMockStorage();
+        const txn = await createGapFillTxn(storage);
+        const fragmentIdComparer = new FragmentIdComparer([]);
+        const relationWriter = new RelationWriter({
+            roomId, fragmentIdComparer, ownUserId: alice,
+        });
+        const gapWriter = new GapWriter({
+            roomId, storage, fragmentIdComparer, relationWriter
+        });
+        const memberWriter = new MemberWriter(roomId);
+        const syncWriter = new SyncWriter({
+            roomId,
+            fragmentIdComparer,
+            memberWriter,
+            relationWriter
+        });
+        return { storage, txn, fragmentIdComparer, gapWriter, syncWriter, timelineMock: new TimelineMock() };
+    }
+
+    async function syncAndWrite(mocks, { previous, limit } = {}) {
+        const {txn, timelineMock, syncWriter, fragmentIdComparer} = mocks;
+        const syncResponse = timelineMock.sync(previous?.next_batch, limit);
+        const {newLiveKey} = await syncWriter.writeSync(syncResponse, false, false, txn, logger);
+        syncWriter.afterSync(newLiveKey);
+        return {
+            syncResponse,
+            fragmentEntry: newLiveKey ? FragmentBoundaryEntry.start(
+                await txn.timelineFragments.get(roomId, newLiveKey.fragmentId),
+                fragmentIdComparer,
+            ) : null,
+        };
+    }
+
+    async function backfillAndWrite(mocks, fragmentEntry, limit) {
+        const {txn, timelineMock, gapWriter} = mocks;
+        const messageResponse = timelineMock.messages(fragmentEntry.token, undefined, fragmentEntry.direction.asApiString(), limit);
+        await gapWriter.writeFragmentFill(fragmentEntry, messageResponse, txn, logger);
+    }
+
+    async function allFragmentEvents(mocks, fragmentId) {
+        const {txn} = mocks;
+        const entries = await txn.timelineEvents.eventsAfter(roomId, new EventKey(fragmentId, KeyLimits.minStorageKey));
+        return entries.map(e => e.event);
+    }
+
+    async function fetchFragment(mocks, fragmentId) {
+        const {txn} = mocks;
+        return txn.timelineFragments.get(roomId, fragmentId);
+    }
+
+    function assertFilledLink(assert, fragment1, fragment2) {
+        assert.equal(fragment1.nextId, fragment2.id);
+        assert.equal(fragment2.previousId, fragment1.id);
+        assert.equal(fragment1.nextToken, null);
+        assert.equal(fragment2.previousToken, null);
+    }
+
+    function assertGapLink(assert, fragment1, fragment2) {
+        assert.equal(fragment1.nextId, fragment2.id);
+        assert.equal(fragment2.previousId, fragment1.id);
+        assert.notEqual(fragment2.previousToken, null);
+    }
+
+    return {
+        "Backfilling after one sync": async assert => {
+            const mocks = await setup();
+            const { timelineMock } = mocks;
+            timelineMock.append(30);
+            const {fragmentEntry} = await syncAndWrite(mocks);
+            await backfillAndWrite(mocks, fragmentEntry, 10);
+            const events = await allFragmentEvents(mocks, fragmentEntry.fragmentId);
+            assert.deepEqual(events.map(e => e.event_id), eventIds(10, 30));
+            await mocks.txn.complete();
+        },
+        "Backfilling a fragment that is expected to close a gap, and does": async assert => {
+            const mocks = await setup();
+            const { timelineMock } = mocks;
+            timelineMock.append(10);
+            const {syncResponse, fragmentEntry: firstFragmentEntry} = await syncAndWrite(mocks, { limit: 10 });
+            timelineMock.append(15);
+            const {fragmentEntry: secondFragmentEntry} = await syncAndWrite(mocks, { previous: syncResponse, limit: 10 });
+            await backfillAndWrite(mocks, secondFragmentEntry, 10);
+
+            const firstFragment = await fetchFragment(mocks, firstFragmentEntry.fragmentId);
+            const secondFragment = await fetchFragment(mocks, secondFragmentEntry.fragmentId);
+            assertFilledLink(assert, firstFragment, secondFragment)
+            const firstEvents = await allFragmentEvents(mocks, firstFragmentEntry.fragmentId);
+            assert.deepEqual(firstEvents.map(e => e.event_id), eventIds(0, 10));
+            const secondEvents = await allFragmentEvents(mocks, secondFragmentEntry.fragmentId);
+            assert.deepEqual(secondEvents.map(e => e.event_id), eventIds(10, 25));
+            await mocks.txn.complete();
+        },
+        "Backfilling a fragment that is expected to close a gap, but doesn't yet": async assert => {
+            const mocks = await setup();
+            const { timelineMock } = mocks;
+            timelineMock.append(10);
+            const {syncResponse, fragmentEntry: firstFragmentEntry} = await syncAndWrite(mocks, { limit: 10 });
+            timelineMock.append(20);
+            const {fragmentEntry: secondFragmentEntry} = await syncAndWrite(mocks, { previous: syncResponse, limit: 10 });
+            await backfillAndWrite(mocks, secondFragmentEntry, 10);
+
+            const firstFragment = await fetchFragment(mocks, firstFragmentEntry.fragmentId);
+            const secondFragment = await fetchFragment(mocks, secondFragmentEntry.fragmentId);
+            assertGapLink(assert, firstFragment, secondFragment)
+            const firstEvents = await allFragmentEvents(mocks, firstFragmentEntry.fragmentId);
+            assert.deepEqual(firstEvents.map(e => e.event_id), eventIds(0, 10));
+            const secondEvents = await allFragmentEvents(mocks, secondFragmentEntry.fragmentId);
+            assert.deepEqual(secondEvents.map(e => e.event_id), eventIds(10, 30));
+            await mocks.txn.complete();
+        },
+        "Receiving a sync with the same events as the current fragment does not create infinite link": async assert => {
+            const mocks = await setup();
+            const { txn, timelineMock } = mocks;
+            timelineMock.append(10);
+            const {syncResponse, fragmentEntry: fragmentEntry} = await syncAndWrite(mocks, { limit: 10 });
+            // Mess with the saved token to receive old events in backfill
+            fragmentEntry.token = syncResponse.next_batch;
+            txn.timelineFragments.update(fragmentEntry.fragment);
+            await backfillAndWrite(mocks, fragmentEntry, 10);
+
+            const fragment = await fetchFragment(mocks, fragmentEntry.fragmentId);
+            assert.notEqual(fragment.nextId, fragment.id);
+            assert.notEqual(fragment.previousId, fragment.id);
+            await mocks.txn.complete();
+        },
+        "An event received by sync does not interrupt backfilling": async assert => {
+            const mocks = await setup();
+            const { timelineMock } = mocks;
+            timelineMock.append(10);
+            const {syncResponse, fragmentEntry: firstFragmentEntry} = await syncAndWrite(mocks, { limit: 10 });
+            timelineMock.append(11);
+            const {fragmentEntry: secondFragmentEntry} = await syncAndWrite(mocks, { previous: syncResponse, limit: 10 });
+            timelineMock.insertAfter(eventId(9), 5);
+            await backfillAndWrite(mocks, secondFragmentEntry, 10);
+
+            const firstEvents = await allFragmentEvents(mocks, firstFragmentEntry.fragmentId);
+            assert.deepEqual(firstEvents.map(e => e.event_id), eventIds(0, 10));
+            const secondEvents = await allFragmentEvents(mocks, secondFragmentEntry.fragmentId);
+            assert.deepEqual(secondEvents.map(e => e.event_id), [...eventIds(21,26), ...eventIds(10, 21)]);
+            const firstFragment = await fetchFragment(mocks, firstFragmentEntry.fragmentId);
+            const secondFragment = await fetchFragment(mocks, secondFragmentEntry.fragmentId);
+            assertFilledLink(assert, firstFragment, secondFragment)
+            await mocks.txn.complete();
+        }
+    }
+}
diff --git a/src/matrix/storage/idb/QueryTarget.ts b/src/matrix/storage/idb/QueryTarget.ts
index e3b77810..3ab33a6b 100644
--- a/src/matrix/storage/idb/QueryTarget.ts
+++ b/src/matrix/storage/idb/QueryTarget.ts
@@ -15,6 +15,7 @@ limitations under the License.
 */
 
 import {iterateCursor, DONE, NOT_DONE, reqAsPromise} from "./utils";
+import {Transaction} from "./Transaction";
 
 type Reducer<A,B> = (acc: B, val: A) => B
 
@@ -31,9 +32,19 @@ interface QueryTargetInterface<T> {
 
 export class QueryTarget<T> {
     protected _target: QueryTargetInterface<T>;
+    protected _transaction: Transaction;
 
-    constructor(target: QueryTargetInterface<T>) {
+    constructor(target: QueryTargetInterface<T>, transaction: Transaction) {
         this._target = target;
+        this._transaction = transaction;
+    }
+
+    get idbFactory(): IDBFactory {
+        return this._transaction.idbFactory;
+    }
+
+    get IDBKeyRange(): typeof IDBKeyRange {
+        return this._transaction.IDBKeyRange;
     }
 
     _openCursor(range?: IDBQuery, direction?: IDBCursorDirection): IDBRequest<IDBCursorWithValue | null> {
@@ -155,11 +166,11 @@ export class QueryTarget<T> {
      */
     async findExistingKeys(keys: IDBValidKey[], backwards: boolean, callback: (key: IDBValidKey, found: boolean) => boolean): Promise<void> {
         const direction = backwards ? "prev" : "next";
-        const compareKeys = (a, b) => backwards ? -indexedDB.cmp(a, b) : indexedDB.cmp(a, b);
+        const compareKeys = (a, b) => backwards ? -this.idbFactory.cmp(a, b) : this.idbFactory.cmp(a, b);
         const sortedKeys = keys.slice().sort(compareKeys);
         const firstKey = backwards ? sortedKeys[sortedKeys.length - 1] : sortedKeys[0];
         const lastKey = backwards ? sortedKeys[0] : sortedKeys[sortedKeys.length - 1];
-        const cursor = this._target.openKeyCursor(IDBKeyRange.bound(firstKey, lastKey), direction);
+        const cursor = this._target.openKeyCursor(this.IDBKeyRange.bound(firstKey, lastKey), direction);
         let i = 0;
         let consumerDone = false;
         await iterateCursor(cursor, (value, key) => {
diff --git a/src/matrix/storage/idb/Storage.ts b/src/matrix/storage/idb/Storage.ts
index 6f7f4c90..72be55ce 100644
--- a/src/matrix/storage/idb/Storage.ts
+++ b/src/matrix/storage/idb/Storage.ts
@@ -24,12 +24,15 @@ const WEBKITEARLYCLOSETXNBUG_BOGUS_KEY = "782rh281re38-boguskey";
 export class Storage {
     private _db: IDBDatabase;
     private _hasWebkitEarlyCloseTxnBug: boolean;
+
     readonly logger: BaseLogger;
+    readonly idbFactory: IDBFactory
     readonly IDBKeyRange: typeof IDBKeyRange;
     readonly storeNames: typeof StoreNames;
 
-    constructor(idbDatabase: IDBDatabase, _IDBKeyRange: typeof IDBKeyRange, hasWebkitEarlyCloseTxnBug: boolean, logger: BaseLogger) {
+    constructor(idbDatabase: IDBDatabase, idbFactory: IDBFactory, _IDBKeyRange: typeof IDBKeyRange, hasWebkitEarlyCloseTxnBug: boolean, logger: BaseLogger) {
         this._db = idbDatabase;
+        this.idbFactory = idbFactory;
         this.IDBKeyRange = _IDBKeyRange;
         this._hasWebkitEarlyCloseTxnBug = hasWebkitEarlyCloseTxnBug;
         this.storeNames = StoreNames;
diff --git a/src/matrix/storage/idb/StorageFactory.ts b/src/matrix/storage/idb/StorageFactory.ts
index 38b48a17..59c988a0 100644
--- a/src/matrix/storage/idb/StorageFactory.ts
+++ b/src/matrix/storage/idb/StorageFactory.ts
@@ -71,7 +71,7 @@ export class StorageFactory {
 
         const hasWebkitEarlyCloseTxnBug = await detectWebkitEarlyCloseTxnBug(this._idbFactory);
         const db = await openDatabaseWithSessionId(sessionId, this._idbFactory, log);
-        return new Storage(db, this._IDBKeyRange, hasWebkitEarlyCloseTxnBug, log.logger);
+        return new Storage(db, this._idbFactory, this._IDBKeyRange, hasWebkitEarlyCloseTxnBug, log.logger);
     }
 
     delete(sessionId: string): Promise<IDBDatabase> {
diff --git a/src/matrix/storage/idb/Store.ts b/src/matrix/storage/idb/Store.ts
index 84e02e60..9c350b98 100644
--- a/src/matrix/storage/idb/Store.ts
+++ b/src/matrix/storage/idb/Store.ts
@@ -133,16 +133,8 @@ class QueryTargetWrapper<T> {
 }
 
 export class Store<T> extends QueryTarget<T> {
-    private _transaction: Transaction;
-
     constructor(idbStore: IDBObjectStore, transaction: Transaction) {
-        super(new QueryTargetWrapper<T>(idbStore));
-        this._transaction = transaction;
-    }
-
-    get IDBKeyRange() {
-        // @ts-ignore
-        return this._transaction.IDBKeyRange;
+        super(new QueryTargetWrapper<T>(idbStore), transaction);
     }
 
     get _idbStore(): QueryTargetWrapper<T> {
@@ -150,7 +142,7 @@ export class Store<T> extends QueryTarget<T> {
     }
 
     index(indexName: string): QueryTarget<T> {
-        return new QueryTarget<T>(new QueryTargetWrapper<T>(this._idbStore.index(indexName)));
+        return new QueryTarget<T>(new QueryTargetWrapper<T>(this._idbStore.index(indexName)), this._transaction);
     }
 
     put(value: T, log?: LogItem): void {
diff --git a/src/matrix/storage/idb/Transaction.ts b/src/matrix/storage/idb/Transaction.ts
index cd68457c..9de4caf2 100644
--- a/src/matrix/storage/idb/Transaction.ts
+++ b/src/matrix/storage/idb/Transaction.ts
@@ -65,6 +65,10 @@ export class Transaction {
         this._writeErrors = [];
     }
 
+    get idbFactory(): IDBFactory {
+        return this._storage.idbFactory;
+    }
+
     get IDBKeyRange(): typeof IDBKeyRange {
         return this._storage.IDBKeyRange;
     }
diff --git a/src/mocks/TimelineMock.ts b/src/mocks/TimelineMock.ts
new file mode 100644
index 00000000..d38e37cc
--- /dev/null
+++ b/src/mocks/TimelineMock.ts
@@ -0,0 +1,263 @@
+import {createEvent, withTextBody, withSender} from "./event.js";
+import {TimelineEvent} from "../matrix/storage/types";
+
+export const TIMELINE_START_TOKEN = "timeline_start";
+
+export function eventId(i: number): string {
+    return `$event${i}`;
+}
+
+/** `from` is included, `to` is excluded */
+export function eventIds(from: number, to: number): string[] {
+    return [...Array(to-from).keys()].map(i => eventId(i + from));
+}
+
+export class TimelineMock {
+    private _counter: number;
+    private _dagOrder: TimelineEvent[];
+    private _syncOrder: TimelineEvent[];
+    private _defaultSender: string;
+
+    constructor(defaultSender: string) {
+        this._counter = 0;
+        this._dagOrder = [];
+        this._syncOrder = [];
+        this._defaultSender = defaultSender;
+    }
+
+    _defaultEvent(id: string): TimelineEvent {
+        return withTextBody(`This is event ${id}`, withSender(this._defaultSender, createEvent("m.room.message", id)));
+    }
+
+    _createEvent(func?: (eventId: string) => TimelineEvent): TimelineEvent {
+        const id = eventId(this._counter++);
+        return func ? func(id) : this._defaultEvent(id);
+    }
+
+    _createEvents(n: number, func?: (eventId: string) => TimelineEvent) {
+        const events: TimelineEvent[] = [];
+        for (let i = 0; i < n; i++) {
+            events.push(this._createEvent(func));
+        }
+        return events;
+    }
+
+    insertAfter(token: string, n: number, func?: (eventId: string) => TimelineEvent) {
+        const events = this._createEvents(n, func);
+        const index = this._findIndex(token, "f", this._dagOrder);
+        this._dagOrder.splice(index, 0, ...events);
+        this._syncOrder.push(...events);
+        return events[events.length - 1]?.event_id;
+    }
+
+    append(n: number, func?: (eventId: string) => TimelineEvent) {
+        const events = this._createEvents(n, func);
+        this._dagOrder.push(...events);
+        this._syncOrder.push(...events);
+        return events[events.length - 1]?.event_id;
+    }
+
+    _getStep(direction: "f" | "b") : 1 | -1 { 
+        return direction === "f" ? 1 : -1;
+    }
+
+    _findIndex(token: string, direction: "f" | "b", eventOrdering: TimelineEvent[]): number {
+        const step = this._getStep(direction);
+        if (token === TIMELINE_START_TOKEN) {
+            const firstSyncEvent = this._syncOrder[0];
+            if (!firstSyncEvent) {
+                // We have no events at all. Wherever you start looking,
+                // you'll stop looking right away. Zero works as well as anything else.
+                return 0;
+            }
+            const orderIndex = eventOrdering.findIndex(e => e.event_id === firstSyncEvent.event_id);
+            return orderIndex;
+        } 
+        // All other tokens are (non-inclusive) event indices
+        const index = eventOrdering.findIndex(e => e.event_id === token);
+        if (index === -1) {
+            // We didn't find this event token at all. What are we
+            // even looking at?
+            throw new Error("Invalid token passed to TimelineMock");
+        }
+        return index + step;
+    }
+
+    messages(begin: string, end: string | undefined, direction: "f" | "b", limit: number = 10) {
+        const step = this._getStep(direction);
+        let index = this._findIndex(begin, direction, this._dagOrder);
+        const chunk: TimelineEvent[] = [];
+        for (; limit > 0 && index >= 0 && index < this._dagOrder.length; index += step, limit--) {
+            if (this._dagOrder[index].event_id === end) {
+                break;
+            }
+            chunk.push(this._dagOrder[index]);
+        }
+        return {
+            start: begin,
+            end: chunk[chunk.length - 1]?.event_id || begin,
+            chunk,
+            state: []
+        };
+    }
+
+    context(eventId: string, limit: number = 10) {
+        if (limit <= 0) {
+            throw new Error("Cannot fetch zero or less events!");
+        }
+        let eventIndex = this._dagOrder.findIndex(e => e.event_id === eventId);
+        if (eventIndex === -1) {
+            throw new Error("Fetching context for unknown event");
+        }
+        const event = this._dagOrder[eventIndex];
+        let offset = 1;
+        const eventsBefore: TimelineEvent[] = [];
+        const eventsAfter: TimelineEvent[] = [];
+        while (limit !== 0 && (eventIndex - offset >= 0 || eventIndex + offset < this._dagOrder.length)) {
+            if (eventIndex - offset >= 0) {
+                eventsBefore.push(this._dagOrder[eventIndex - offset]);
+                limit--;
+            }
+            if (limit !== 0 && eventIndex + offset < this._dagOrder.length) {
+                eventsAfter.push(this._dagOrder[eventIndex + offset]);
+                limit--;
+            }
+            offset++;
+        }
+        return {
+            start: eventsBefore[eventsBefore.length - 1]?.event_id || eventId,
+            end: eventsAfter[eventsAfter.length - 1]?.event_id || eventId,
+            event,
+            events_before: eventsBefore,
+            events_after: eventsAfter,
+            state: []
+        };
+    }
+
+    sync(since?: string, limit: number = 10) {
+        const startAt = since ? this._findIndex(since, "f", this._syncOrder) : 0;
+        const index = Math.max(this._syncOrder.length - limit, startAt);
+        const limited = this._syncOrder.length - startAt > limit;
+        const events: TimelineEvent[] = [];
+        for(let i = index; i < this._syncOrder.length; i++) {
+            events.push(this._syncOrder[i]);
+        }
+        return {
+            next_batch: events[events.length - 1]?.event_id || since || TIMELINE_START_TOKEN,
+            timeline: {
+                prev_batch: events[0]?.event_id || since || TIMELINE_START_TOKEN,
+                events,
+                limited
+            }
+        }
+    }
+}
+
+export function tests() {
+    const SENDER = "@alice:hs.tdl";
+
+    return {
+        "Append events are returned via sync": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(10);
+            const syncResponse = timeline.sync();
+            const events = syncResponse.timeline.events.map(e => e.event_id);
+            assert.deepEqual(events, eventIds(0, 10));
+            assert.equal(syncResponse.timeline.limited, false);
+        },
+        "Limiting a sync properly limits the returned events": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(20);
+            const syncResponse = timeline.sync(undefined, 10);
+            const events = syncResponse.timeline.events.map(e => e.event_id);
+            assert.deepEqual(events, eventIds(10, 20));
+            assert.equal(syncResponse.timeline.limited, true);
+        },
+        "The context endpoint returns messages in DAG order around an event": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const context = timeline.context(eventId(15));
+            assert.equal(context.event.event_id, eventId(15));
+            assert.deepEqual(context.events_before.map(e => e.event_id).reverse(), eventIds(10, 15));
+            assert.deepEqual(context.events_after.map(e => e.event_id), eventIds(16, 21));
+        },
+        "The context endpoint returns the proper number of messages": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            for (const i of new Array(29).keys()) {
+                const middleFetch = timeline.context(eventId(15), i + 1);
+                assert.equal(middleFetch.events_before.length + middleFetch.events_after.length, i + 1);
+                const startFetch = timeline.context(eventId(1), i + 1);
+                assert.equal(startFetch.events_before.length + startFetch.events_after.length, i + 1);
+                const endFetch = timeline.context(eventId(28), i + 1);
+                assert.equal(endFetch.events_before.length + endFetch.events_after.length, i + 1);
+            }
+        },
+        "The previous batch from a sync returns the previous events": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(20);
+            const sync = timeline.sync(undefined, 10);
+            const messages = timeline.messages(sync.timeline.prev_batch, undefined, "b");
+            const events = messages.chunk.map(e => e.event_id).reverse();
+            assert.deepEqual(events, eventIds(0, 10));
+        },
+        "Two consecutive message fetches are continuous if no new events are inserted": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+
+            const sync = timeline.sync(undefined, 10);
+            const messages1 = timeline.messages(sync.timeline.prev_batch, undefined, "b");
+            const events1 = messages1.chunk.map(e => e.event_id).reverse();
+            assert.deepEqual(events1, eventIds(10, 20));
+
+            const messages2 = timeline.messages(messages1.end, undefined, "b");
+            const events2 = messages2.chunk.map(e => e.event_id).reverse();
+            assert.deepEqual(events2, eventIds(0, 10));
+        },
+        "Two consecutive message fetches detect newly inserted event": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+
+            const messages1 = timeline.messages(eventId(20), undefined, "b", 10);
+            const events1 = messages1.chunk.map(e => e.event_id).reverse();
+            assert.deepEqual(events1, eventIds(10, 20));
+            timeline.insertAfter(eventId(9), 1);
+
+            const messages2 = timeline.messages(eventId(10), undefined, "b", 10);
+            const events2 = messages2.chunk.map(e => e.event_id).reverse();
+            const expectedEvents2 = eventIds(1, 10);
+            expectedEvents2.push(eventId(30));
+            assert.deepEqual(events2, expectedEvents2);
+        },
+        "A sync that receives no events has the same next batch as it started with": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(10);
+            const sync1 = timeline.sync();
+            const sync2 = timeline.sync(sync1.next_batch);
+            assert.equal(sync1.next_batch, sync2.next_batch);
+        }, 
+        "An event inserted at the staart still shows up in a sync": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const sync1 = timeline.sync(undefined, 10);
+            const sync2 = timeline.sync(sync1.next_batch, 10)
+            assert.deepEqual(sync2.timeline.events, []);
+            assert.equal(sync2.timeline.limited, false);
+
+            timeline.insertAfter(TIMELINE_START_TOKEN, 1);
+            const sync3 = timeline.sync(sync2.next_batch, 10)
+            const events = sync3.timeline.events.map(e => e.event_id);
+            assert.deepEqual(events, [eventId(30)]);
+        },
+        "An event inserted at the start does not show up in a non-overlapping message fetch": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const sync1 = timeline.sync(undefined, 10);
+            const messages1 = timeline.messages(sync1.timeline.prev_batch, undefined, "f", 10);
+
+            timeline.insertAfter(TIMELINE_START_TOKEN, 1);
+            const messages2 = timeline.messages(sync1.timeline.prev_batch, undefined, "f", 10);
+            assert.deepEqual(messages1.chunk, messages2.chunk);
+        },
+    }
+}