If I delete a message in WhatsApp with one checkmark, what will the recipient see - briefly?
If you delete a message in WhatsApp that has only one checkmark (sent but not yet delivered), the recipient will not see any notification or indication of the deleted message. The message is considered unsent and thus does not appear in their chat history.
If I delete a message in WhatsApp with one checkmark, what will the recipient see - in detail?
When you delete a message in WhatsApp that has been sent but not yet delivered (indicated by one checkmark), the behavior of the deletion depends on whether the recipient has seen the notification or not.
If the recipient has not seen the notification, they will simply never receive the message. The message will be permanently deleted from both your device and the recipient's device before it is even delivered. As a result, the recipient will not see any trace of the message, nor will they know that a message was ever sent or subsequently deleted.
However, if the recipient has seen the notification but has not yet opened WhatsApp to read the message, deleting the message from your end will remove it from their phone as well. Again, they will not see any trace of the message, and there will be no indication that a message was sent and then deleted.
In summary, if you delete a message in WhatsApp with one checkmark before the recipient has seen or interacted with it, the recipient will see nothing. The message will be permanently removed from both devices without any notification to the recipient.