I had this issue about 6 to 12 months ago and it got solved by a software update. It is back!
When I double click on a message in my messages list in Outlook, the message opens ok but the email window being opened is NOT focused. So, for example, if I type CTRL-R to reply, well, it does not. It's not even focused on the message list neither because it is replying.
Currently running Version 2110 build 14527.20234.
