Present: undocumented behavior when divisor == 0
Relevant code:
https://gitlab.freedesktop.org/xorg/xserver/-/blob/master/present/present.c?ref_type=heads#L169 https://gitlab.freedesktop.org/xorg/xserver/-/blob/master/present/present.c?ref_type=heads#L275
Apparently divisor == 0
is used to mean async presentation, which is not documented.