By observation, it appears that disabling vsync occurs on any port output whatsoever, as long as NMI isn't blocking it.
By observation, it appears that disabling vsync occurs on any port output whatsoever, as long as NMI isn't blocking it.

TomHarte/CLK,TomHarte/CLK,TomHarte/CLK,TomHarte/CLK,TomHarte/CLK