Movement Deltas on the Firefox DOM

What I did was added a non-scriptable method to an nsIDOMWindow that saves the X/Y so that the next mouse event could get it, as well as attributes for the previous x/y.

(in nsIDOMWindow.idl)
[noscript] void saveScreenPosition(in long x, in long y);
readonly attribute long lastScreenX;
readonly attribute long lastScreenY;

Then, in nsGlobalWindow, that function simply sets the two members. When an nsDOMMouseEvent is created, it stores the movement deltas based on the previous/current values, then tells the nsGlobalWindow to save the current one.

(In nsDOMMouseEvent’s constructor, mView is the nsIDOMWindow)
if (mView) {
PRInt32 screenPosX, screenPosY, lastScreenPosX, lastScreenPosY;
GetScreenX(&screenPosX);
GetScreenY(&screenPosY);
mView->GetLastScreenX(&lastScreenPosX);
mView->GetLastScreenY(&lastScreenPosY);
mMovement.x = screenPosX – lastScreenPosX;
mMovement.y = screenPosY – lastScreenPosY;
mView->SaveScreenPosition(screenPosX, screenPosY);
}

And that was pretty much it, success.