cursors: Fix cursor batching calculation

The cursor batching algorithm was designed so that the overall channel
message rate remains lower than an upper bound limit, and the algorithm
expects the batching to multiply the batch interval by the number of
members, not the number of members minus one as we're currently doing.

Signed-off-by: Lewis Marshall <lewis.marshall@ably.com>

src/Cursors.test.ts

@@ -116,7 +116,7 @@ describe('Cursors', () => {
       vi.spyOn(channel.presence, 'get').mockImplementation(createPresenceCount(2));
       await cursors['onPresenceUpdate']();
       expect(batching.shouldSend).toBeTruthy();
-      expect(batching.batchTime).toEqual(100);
+      expect(batching.batchTime).toEqual(200);
     });
 
     it<CursorsTestContext>('batchTime is updated when multiple people are present', async ({
@@ -126,7 +126,7 @@ describe('Cursors', () => {
     }) => {
       vi.spyOn(channel.presence, 'get').mockImplementation(createPresenceCount(2));
       await cursors['onPresenceUpdate']();
-      expect(batching.batchTime).toEqual(100);
+      expect(batching.batchTime).toEqual(200);
     });
 
     describe('pushCursorPosition', () => {

src/Cursors.ts

@@ -76,7 +76,7 @@ export default class Cursors extends EventEmitter<CursorsEventMap> {
     const channel = this.getChannel();
     const cursorsMembers = await channel.presence.get();
     this.cursorBatching.setShouldSend(cursorsMembers.length > 1);
-    this.cursorBatching.setBatchTime((cursorsMembers.length - 1) * this.options.outboundBatchInterval);
+    this.cursorBatching.setBatchTime(cursorsMembers.length * this.options.outboundBatchInterval);
   }
 
   private isUnsubscribed() {