XFCE: M-Tab, active windows and „Always on Top“

I run XFCE under X11 with focus policy „focus follows mouse“. What happens when I open an application, create a new (second) window in it, set it to „Always on Top“ from window menu, move the mouse pointer into it, and press M-Tab (aka Alt-Tab)? It has turned out in my experiments that different applications behave differently in this respect. For Okular, Thunar and Firefox, pressing M-Tab results in the first window becoming active; for XFCE Terminal and Emacs, the second window remains active as long as the mouse pointer is in it, and there seems to be no way to make the first window active using the keyboard. What does this behavior depend on? How could I make Emacs behave like Okular so that I don't have to reach for the mouse to make the non-always-on-top window active? My version of XFCE is 4.18.