It should, but it isn't...
I don't use Windows, so I'd basically have to eyeball it, which I'm happy to do if I have guinea pigs. Otherwise, you should ask Robert.
@RobertBColton when you're less busy, any chance you could take a look at this? thanks!
Use the function, not the global game settings. There's tons of game settings that haven't been implemented, I was going to rework that at some point.
Wow, why is the global game setting not just using the function? Is it taken into account while setting up the window?
Nevermind, there is no actual function, the setting does work but he's right that the maximize button is for some reason disabled. It's probably the window style that's being set.
Edit: Can confirm in GMS 1.4 that the setting should allow the maximized button.
Edit 2: The setting is really hard to find because we name it 3 different things including allowResize, isSizable, allowWindowResize, and we also have an unused variable resizable.
Anyway, isSizable does set the maximize box style too so I don't know why it does appear: