dix: leave last.valuators alone on slave switch
Terms: dev->last.valuator is the last value given to us by the driver dev->valuator.axisVal is the last value sent to the client dev->last.scroll is the abs value of the scroll axis as given by the driver, used for button emulation calculation (and the remainder)
This function updates the device's last.valuator state based on the current master axis state. This way, relative motion continues fluidly when switching between devices. Before mouse 2 comes into effect, it's valuator state is updated to wherever the pointer currently is so the relative event applies on top of that.
This can only work for x/y axes, all other axes aren't guaranteed to have the same meaning and/or may not be present:
- xtest device: no valuator 2
- mouse: valuator 2 is horizontal scroll axis
- tablet: valuator 2 is pressure
Scaling the current value from the pressure range into the range for horizontal scrolling makes no sense. And it causes scroll jumps:
- scroll down, last.valuator == axisVal == 20
- xdotool click 1, the XTest device doesn't have that valuator
- scroll up
- updateSlaveDeviceCoords reset last.valuator to 0 (axisVal == 20)
- DeviceClassesChangedEvent includes value 20 for the axis
- event is processed, last.value changes from 0 to -1
- axisVal is updated to -1, causing a jump of -21
The same applies when we switch from tablet to mouse wheel if the pressure value is 0 on proximity out (basically guaranteed). So let's drop this code altogether and only leave the scaling for the relative x/y motion.