1. Run any flutter app on web since Web trackpad pan engine#36346
2. Start a one-finger drag on the trackpad
3. Put a second finger on the trackpad to start scrolling
4. If this is done fast enough, it will cause an assertion failure

See #117881 (comment) for log

The root cause is that the trackpad events are sent with the same device ID as mouse events (_mouseDeviceId = -1), so if a trackpad scroll begins when the pointer is down, a PointerMoveEvent might be synthesized if the location has changed (moving fast enough).

A different device ID should be used for trackpad events, this is what's been done on other platforms (_trackpadDeviceId = -2?).