Editor Window Will Not Maximize

[Alienchild]

There is no such thing as a "computers GUI". What you are referring to is the Windows GUI (or Mac/Linux). They use their own buttons, windows and such, and them being rendered is done by Windows. The GUI in the leveleditor is most likely the game engine rendering opengl elements (quads), and as such when you change the resolution you need to reset the renderer, hence you have to restart the engine.

It's just a guess on my part though.