Reply to email - can I stop the reply popup window?

When clicking on a received email, in the viewing panel on the right allows me to view the message details. In this view, I have always been able to click the reply button and, in the same view panel, could type my reply above. Recently, clicking on the reply button, I now have a reply popup window - not sure why this is happening. How can I stop this popup and reply as before - from the viewing panel?
Many thanks

eM Client has never had that feature. Replies have always been in a pop-up window.

Gary - I apologise for being such a thicky - I have just realised that because the window has all of a sudden popped up minimised and not full screen as before - this is what confused me! have corrected now 8o)
Thank you