This now looks like an SDL bug. The coordinates from SDL_PollEvent are already wrong: event->motion.xrel is -10824 at the left edge of the tablet and +10824 at the right edge, i.e. absolute position rather than relative motion; and event->motion.x increases/decreases according to xrel while being clamped to [0, 1279], which presumably corresponds to the selected 1280x720 resolution. (1366x768 is the native resolution in this laptop.)
The Barrage game has no such problem with the tablet and I see it gets the coordinates from SDL_GetMouseState rather than from SDL_PollEvent. I didn't find anything relevant in the src:libsdl1.2 bug list.
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