[Context for other readers: "Scanline Sync" refers to the brand new RTSS Scanline Sync, a frame rate capping method that can steer tearing to exact positions on the screen, explained in this thread -- the new low-lag VSYNC ON mode that it uses, but it is also capable of running at framerates twice the Hz, with two tearlines, one of which can be steered offscreen.]
It's a trick question with a trick answer.
--> Logically, one may answer "always the lower one". During VSYNC OFF, the freshest frames are always at the bottom.
--> However, temporally, the top and bottom is the same lag in terms of API-to-photons on most eSports monitor, because of the panel scan-out behavior.
On displays that realtime synchronize panel scanout to cable scanout, the button-to-pixels latency (mouse button to pixels emitted by photons) is identical for top and bottom, because of the sequential display scan-out behavior.
To understand how a display scans, it's always top to bottom.
Here's a 960fps high speed video of 60fps VSYNC ON:
During VSYNC OFF, display scan-out order is this:
So the frameslices being scanned out, are all the same latency, no matter where they are on the screen, as long as the display panel scanout is synchronized to cable scanout.
That said, within each frameslice is a latency gradent, with higher lag for bottom edge of frameslice (lag is always highest right above the tearline, and lowest right below tearline, but this equally applies to all tearlines. So you have a situation of "+0ms" scanout lag adder for top edge of frameslices (in scanline sync framerate 2x Hz, lowest lag is top edge & just below screen middle). And a situation of "+(1/2 refresh cycle)" scanout lag adder for bottom edge of frameslices (in scanline sync framerate 2x Hz, highest lag is just above screen middle, and just above bottom screen edge)
For more reading, see
Understanding Display Scan-Out Lag With High Speed Video.
How's that for a full technical answer to a tricky question?
