This then goes to the Os team, not to P96. Apparently, there is a bug in the mouse blanker, it should not set the Viewport argument to NULL in which case it only blanks the mouse on the first viewport in the view chain, but it rather should transmit the viewport of the frontmost screen as such:
Code:
#define BlankMouse() ChangeSprite(IntuitionBase->FirstScreen?(&IntuitionBase->FirstScreen->ViewPort):NULL,&blankMouse,(APTR)blankData);
but that is really beyond P96 possibilities to fix.