We currently have a test in modify.js that starts a SP with initialSync=true, then after the collectionCopy is over, it modifies the SP to set initialSync=false and then adds a tumbling window to the modified pipeline.

When the above test is modified so that the added tumbling window is a processing time window, it causes an off-by-one error where the last message in inputAfterModifyBeforeStart is not processed as part of the window.

Investigate this behavior and add a testcase like the following to test the addition of a tumbling window with processingTIme mode.

```
modifiedPipeline: [
{$replaceRoot: {newRoot: "$fullDocument"}},
{
$tumblingWindow: {
interval:

,
pipeline: [{$group: {_id: "group1", sum: {$sum: "$c"}}}]
}
},
{$set: {initialSync: {$meta: "stream.source.initialSync"}}},
],
```