A way round this problem would to set a velocity threshold below which the on-screen compass reading is not changed/updated - this would mean that if you set a low threshold like 5 mph or whatever, then when the car was stopped you would see the direction you were going when you were going above the threshold (i.e. just before you stopped), you could also have a little indicator to show that this is not a live reading (e.g. make it flash or have a * beside it to show its a last known direction not the current direction).

Note: as GPS has a slight inaccuracy to it, you may appear to be still moving even when stationary due to these errors, which are much reduced now that SA has been turned off. So the threshold can't be 'zero', but it may be able to be lower than say 5mph, for use in a car anything below about 5mph is a good as being stationary, and if the threshold was user adjustable from the front panel then so much the better.