If it says, at the bottom, that the unit is in the terrain, then it is in it, and getting all the benefits that apply.
(at least, that's what I've always thought, and seemed to observe!)
If it doesn't appear graphically that way, it's because the shape of terrain effect doesn't quite match the graphics. Usually it's the opposite to what you have said: something can appear to be in woods, on the corner, and actually still be in open.
Oh - I've never really noticed "partial" effects, except clearly as things get deeper into woods they are harder to see.
GaJ.