The public variable constrainOnDrag in the UIScrollView class is not exposed in the inspector, unlike restrictWithinPanel or disableDragIfFits.
It's a public variable and it seems like it should be available in the inspector as it controls the behavior of the scrollview just like the other public variables related to dragging. I added the following line to UIScrollViewEditor.cs (line 44).
NGUIEditorTools.DrawProperty("Constrain On Drag", serializedObject, "constrainOnDrag");
