DF Pro Trial v9.7.1
Windows 10 20H2
Blender 2.91

Steps to repeat
01. Open Blender 2.91
02. Assign shortcut to the menu command "Toggle Window Fullscreen"
03. Use the shortcut to trigger full application fullscreen.
04. Application starts "jiggle" vilently. Its trying to go into fullscreen but is prevented.

Needed result
DisplayFushion should not intervene with apps going into full frameless mode

Additional info
I noticed today when i use a shortcut from BLender to go through fullscreen, no application frame. The applications starts to "wiggle", its try to go into true fullscreen but something is preventing it. THe weird thing is when i use the menu command, which is same as shortcut it does work. Then when im in the true fullscreen mode, neither does the shortcut or menu command exit this mode.

The shortcut works fine on the main monitor by the way. When i use this shortcut on the second monitor then this behavior starts. It happens every time so it can be repeated.

Ive tried looking in the shortcuts for a perhaps a conflicting shortcut but cant find it. When i exit DF the shortcut works just fine.

Im new to this app, downloaded it this weekend. Quite nice! I was only looking for an option to have trayicons on both monitors, but it has so many options. To much for me but its nice having them i guess.

Ive made a screengrab from this weird behavior and also attached a log file from settings.

PS when you try to open the log file using the button. It returns an error that it cant find the log file?!

I think i found my issue. In the monitor configuration, at the bottom there is boolean Window Management. When i turn this OFF everything works as expected.

Is this perhaps causing an issue because i use shift in the shortcut of Blender?

Im not sure what this function actually should do. It states "hold shift", but it doesnt state in what kind of interaction. Shift + drag does nothing, shift + click does nothing, shift+double click, no difference?!

EDIT
I did noticed shift + click drag to second monitor showed this hint "drag to other monitor to maximize". It showed once but it didnt do anything, it did not maximaze the window. This is with the option ON.

EDIT
now i see how it works. It only popsup when the app is maximized and then dragged, then Shift will prevent the window going into fullscreen on the second monitor. THats is weird because the option in Monitor Configuration tells you the opposite.

See attached screengrab. The tooltip from Window Management contradicts what is actually happening.