Latest version of display routines (drawPixelRaw) seems to draw outside the screenbuffer in some instances (@FManga)
I tracked down this pretty odd bug using a hardware debugger when I saw odd behaviour in the Tracker software (see below)
By adding a memory write breakpoint, I found it was drawPixelRaw writing to memory outside the actual screenbuffer.
I am not pressing any buttons in the video below. That is the drawPixelRaw writing on top of the memory of instrument.wave