Although this is standard behaviour, so this would be classed as a "New Idea", I'd class this as a bug.

When using the "Enhanced Email for Timeline" feature, if you create a new email from a case/opportunity, then choose to pop-out the email to a new window, the "Regarding" and "To" fields are cleared in the popup.

Work around is to press "Save" before popping the email out to a new window.

My idea is that when the pop-out button is pressed, the information from the email which has been drafted should be copied to the new email which appears when it is popped out.