I think testing with Hatari instead of STEem might help here. The current wakestate-code in STEem isn't really emulating the hardware but more of an in-between thing that works with the code that has already been published and tested. I'm quite sure Spectrum 512 has a uniform border color.
I'm guessing left/right is of most interest? I believe I have some code by Evil/DHS that only displays a fullscreen, made for calculating number of visible lines. Both ST (230 byte) and STE-specific (224 byte) line variants.Can anyone suggest a program that displays a static image that extends into the overscan area? Preferably something that lets you specify which image, a picture viewer for example. With demos, there's usually crap flying everywhere and it's really hard to see what's going on.
I think it's important to separate GLUE-MMU wakestates (WS1-WS4, can be tested in software) and Shifter(-MMU) wakestates (not completely researched, but "Spectrum 512 black pixels" is one. Cannot be detected from software, only visually).
GLUE-MMU wakestates only exist on ST, not STE.
Shifter-MMU wakestates, at least some, exists on both ST and STE.