So, as I wondered there, it turns out that the tablet cursor can fall into the crack between my landscape left monitor and portrait right monitor, with the distressing result that the X server jams with the pointer jittering between the two displays. Worse, the keyboard isn’t active, so I can’t switch to a console screen and shoot X in the head.

I’m guessing this is a picket-fence error, where Something Bad Happens when the cursor hits the maximum possible value: X=16704, in this case.

The solution seems to be telling the Wacom driver that the tablet is just slightly wider than it really is, so that the X value can’t ever reach the maximum. Having done this before, the line is already in my xorg.conf, just waiting to be un-commented:

Section "InputDevice"
    Identifier      "Wacom - stylus"
    Driver          "wacom"
    Option          "Device" "/dev/input/wacom"
    Option          "USB" "on"
    Option          "Type" "stylus"
    Option          "Button2" "3"
    Option          "Button3" "2"
    Option          "MMonitor" "off"
    Option          "ScreenNo" "0"
    Option          "BottomX" "16710"
#    Option          "BottomY" "11893"
EndSection

Section "InputDevice"
    Identifier      "Wacom - eraser"
    Option          "Device" "/dev/input/wacom"
    Driver          "wacom"
    Option          "USB" "on"
    Option          "Type" "eraser"
    Option          "MMonitor" "off"
    Option          "ScreenNo" "0"
    Option          "BottomX" "16710"
#    Option          "BottomY" "11893"
EndSection

That seems to work, but the failure was intermittent. We shall see…