Dear Byte99,
Good day! Thank you for posting to Microsoft Community. We are happy to help you!
Are you using the classic version or are you enabled for the new version? The classic version does not have a pop-up button (because it does not support composing in the reading pane)
When you hover over the edges (any edge or corner), the mouse pointer should change and you can drag. Or maximize the window by clicking on the green dot.

If you still have more problem, for using Outlook for Mac, as we have a dedicated support team for it, which is specializing in handling issues related to Outlook for Mac. Therefore, we recommend you that for further assistance please go to Outlook for Mac support.
Your contribution will help more and more users in the community! Hope you all the best!
Sincerely,
Kerry Chen | Microsoft Community Moderator
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