privacy.resistFingerprinting.letterboxing = true
Red ball remains in same space.
Red ball drifts slowly to one side depending on letterbox size.
Does not happen with letterboxing disabled.
the pointer coordinates are being taken from the inner window rather than the viewport. Firefox (not Tor Browser) centers vertically. Both center horizontally.
Create a letterbox that has e.g. 10px matt to the left and 10px matt above and just click the mouse once to watch the red ball move on its own (diagonally and down). When it crosses the x or y threshold, you can see the coordinate is -10 (for example)