Input changes on same scanline every time

[pinobatch]

A program can tell that it's running on RusticNES because the buttons always change right at vblank. The games Spot and Quattro Sports depend on this and will not work reliably if the buttons always change at the start or end of vblank.

To reproduce:

1. Install RusticNES SDL
2. Start the input timing test ROM "Telling LYs" from little things
3. Press all 8 buttons in any order
4. Watch the arrow sprite at the right side

Expect: Arrow moves after each press, followed by "Pass", as on my NES with PowerPak

Actual: Arrow stays invisible of the screen, followed by a message that clearly isn't "Pass":

Incorrect behavior:
Controller state changes
on same line each frame

The Game Boy emulator BGB by beware passes this test. Normally it changes the input on the first scanline of vertical blanking to reduce input latency. But if a program reads input multiple times during a frame, the emulator starts to randomize on which scanline the input changes. This way, the program can't discern that it's being emulated and freeze on a copy protection screen, but it adds one frame of lag. Once the program returns to polling once a frame, BGB returns to the low-latency behavior.

• Related forum topic: https://forums.nesdev.com/viewtopic.php?f=22&t=18998
• Telling LYs tracking bug: Telling LYs tracking bug pinobatch/little-things-gb#3