I can no longer use the Undo command while recording with Pro Tools 5.3.1. I was able to with Pro Tools 5.3

When you undo an edit, and then perform a new edit, the original edit is removed from the undo queue. That?s the basic nature of most undo queues.
In the case of record passes that are undone and removed from the queue, the audio file is also removed from disk. This situation is dangerous during a record pass because the record command doesn’t get posted until you stop recording. Therefore, anything you had undone during the record pass would be removed from the queue as soon as you stopped recording.
In version 5.3.1, we made it so you can?t undo during a record pass. You CAN still perform edits during record. You just have to stop recording to undo those edits. We found this late in 5.3.1 and preventing undo during record was the best option at the time.

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