pretty straight forward transparent option for ALT+MiddleClick. I only query the ALT key for exiting, so you can release the Middle Button to make things more easy (and to prevent accidential scrolls with the ScrollWheel)
TC
____________________________
Code:
!MButton::
CoordMode, Mouse, Screen
MouseGetPos, currentX, currentY, currentWindow
if currentWindow =
return
SetWinDelay, 0
WinSet, Transparent, 50, ahk_id %currentWindow%
Loop
{
GetKeyState, state, LAlt, P
if state = U
break
}
WinSet, Transparent, 255, ahk_id %currentWindow%
return