performance of get scanline
accuracy of get scanline, in the vblank. Mark Rejhon says it's not updating
whether update_scanline_boundaries() works properly. for example, the back edge of the display is wobbly. but what about the front edge

tearing, whether that steep drop still exists
GPU swap delay - adjust so that when holding left click and dragging to the bottom, the top of the green/purple flashing to the left coincides with your mouse
	in the future: will instead change to "click on the top of the flashing green/purple bar"

latency needed for CPU -> GPU. for example, 2 ms on my computer, "time_to_spend_rendering"

WGL_EXT_swap_control_tear: doesn't work on my GPU. maybe on others?

performance of sleep.
Timer, the new high resolution timers in Windows 10
WaitForSingleObject in Windows 10
the monitor active display boundaries in Windows 10

is the claimed monitor Hz ever off by more than 1%?

scanline stability
buffering 1 float is enough to wake up GPU?

mouse input, high DPI. whether 0 is always reachable