Visual inspection for now. I've got an Arduino and a high-speed camera, so my plan for the next step is to send mouse and keyboard events from the Arduino, blinking an LED at the same time, then capture both the LED and the monitor in the video. Then a bit of image analysis. This is the only way to be quantitative and capture all the sources of latency.

The implication of what I was saying is that windows will always show the cursor lining up with the border of a window during resizing.

Would that it were so.

Hmm, are you inspired or were part of https://github.com/google/walt ?

Would you be testing this with a high refresh rate monitor (120hz+) ? Would that matter?

Yes and yes. My son has a 144Hz gaming monitor, and my main monitor for coding is a Dell 4k. We'll test both.

I also believe that is true.

My reason being, I had adjusted the screen brightness using software - but the mouse cursor is still brighter than white anywhere else. When I dragged a window, the cursor turns dim.

Oh. This explains why the cursor flickers briefly and gets drawn partially across multiple monitors when at the split instead of the normal mouse cursor which is drawn only on one monitor at a given time.