That works reasonably well. First tried it in Windows Explorer ... went full screen. After a moment's reflection, that makes sense, since explorer is the desktop .
Tried it again in Firefox ... worked just fine. However, those lines are just a bit too fine for these old eyes. Guess I could go in and play with it, since the .ahk file is part of the download. Have to make the hotkeys adjustable - I hate hotkeys - since those are occupied on my system.
it will only go full screen if the nominated window is also full screen - if you resize the window, then the cross hair window will resize to match
EDIT: I just tested this and it goes full screen on the desktop.
Window spy reports the desktop (in Win7) as 'program manager', so I assume that would explain the behaviour (kind of...)
hotkeys are very personal, so by all means change them. This was a first cut, so I expected some feedback (I rarely use any of the snacks I do so I tend to re-use a standard set of hotkeys)
not sure if there's much you can do about the thickness of the lines. This is really a kludge using group boxes, and I'm pretty sure there's not much scope for customising them.
Actually, you can add a v e r y long literal (eg 5000 asterix) to the horizontal control. This would be thicker than the controls outline, but should't be obtrusive (at least it looks OK ish on my desktop at 1680 X 1050)