I've got a bug for years with the "Menu Bar" in fusion and at the moment I'm using the Build 293.10 Steam Version. It was suggested by NaitorStudios to post on this thread since BugBox is Offline.
The actual thing is that the "Menu Bar" behavior with the "Alt" button is weird. Even if you keep the "Menu Bar" enable or disables or even if you change menus in it or keep it the defaults option the bug happen.
The bug itself is that the Application is "Pausing" or "Slowing" depending on the option of "Run while minimized" and "Run while resizing" is toggle or not when you press the TAB button.
To make the bug happen, change the application window with ALT-TAB for example then come back to the game and just press ALT. The application will pause. You absolutely need to defocus the application and refocus it.
(You don't need any Fullscreen mode or anything)
If you use the option "Run while minimized" and "Run while resizing" in the game properties the game won't "Pause" but the game will be laggy with about 30 FPS visually and practically it output that the framerate is actually about 30-40FPS
I tried a lot of thing to make a workaround it with extension and such but I didn't find any solution yet. A workaround might do for now but it seems that many Fusion game suffer from this and some developers might not even know. (At the moment there is no such way to completely disable the "Menu Bar" it's only possible to not show it and you cannot even remove everything inside of it because it come back if you do so. It's been for years)
Here's an MFA file with the same information in it. https://filetransfer.io/data-package/VMxWn2CX#link
(There is also the attachment here : Attachment 31395
. If the link is not working at the moment that you wanna download the example, message me for the MFA)
Side notes : I've tried other Fusion game like Inexistance and it seems to do the same bug, Then I tried FNaF 3 and it didn't. :o