When in OnSceneGUI related callbacks, Screen.width is 3 pixels too big and Screen.height is 38…
For example if the actual width of the scene view is 700 pixels, Screen.width says 703. And if scene view height were 700 pixels, Screen.height would return 738
I know this question is seven years old now, but in case anyone else got stumped on this like I did, the screen height is returning with the menu and toolbars at the top added for some reason.
To get the actual viewable areas, you can use