I opened a thread about this some time ago :
https://forum.cockos.com/showthread.php?t=236928
There, a user talks about a window manager -bspwm in his case -, with which he manages the issue. I took a look at it, but it seemed rather complex and I haven't had time to delve in the issue, but neither did I realize that it could be indeed a Xfce thing. Anyway, as a Windows user, Xfce is too comfortable for my taste to change it.
I suppose Justin could adjust Reaper's behaviour to enable this feature in other mainstream desktops, but I don't know how much of a hassle it would be for him -or if he will even read this at all -. I hope this can be fixed at some point, since it's definitely a quite useful feature in many scenarios.