What controls the overall background color on Z-machine?

[Draconis (Daniel Stelzer)]

I found this right after posting, of course:

@prevtenet

Interpreter behavior is inconsistent. Parchment sets the border color on executing @erase_window 0 (CLEAR 0), effectively matching the border color to the color of the lower window. This seems like the most sensible expected behavior. Gargoyle, by contrast, does not respect the background color when executing @erase_window at all (which actually seems like a bug); instead, it sets the background color of all windows and borders to the current background color when an input opcode is executed (@read, @read_char). In practice most games with non-default background colors will both clear the screen and subsequently wait for input before changing the background color again, giving either approach similar results, but the inconsistency could cause issues for any particularly psychedelic usages of color.

Which explains the Gargoyle behavior. I’m not sure about Frotz though.