Weird black lines while loading Quartet's border

For some reason, during [*Quartet*](https://makrill.itch.io/quartet)'s initial load, the border briefly flashes with dark lines. Investigate and document why. (Note: if the bugs ends up fixed on that download page, it's still present in [the cracktro](https://www.pouet.net/prod.php?which=87951).)

Tip: the bug reproduces on [Mesen-S](https://github.com/NovaSquirrel/Mesen-S/), which has both a GB and SNES debugger, so it should be easier to figure out.