This is interesting - are you saying it's possible with appropriate synchronization to race the scanout on a modern GPU/display to get true line-by-line display

This is interesting - are you saying it's possible with appropriate synchronization to race the scanout on a modern GPU/display to get true line-by-line display for an emulator and avoid the buffering delay (e.g. 1/4 frame at 120 Hz)? Presumably native graphics and games already need to sync with fixed rate displays to avoid tearing (or buffer and pay a latency price.) The fact that tearing exists would seem to indicate that you can update a frame while it is being scanned out. I imagine that racing the beam (or video signal scanout in the case of non-CRTs) might actually work, especially with VGA where you could set to an appropriate resolution.