The VGA attribute controller is weird

The grabber in Windows 3.1 was improved to save and restore the index register as well, but it does not attempt to restore the flip-flop state, which is significant. The problem with the VGA emulation was that it erroneously applied the flip-flop state to reads from port 3C0h, and Windows 3.1 would save the wrong index register value... but only the second time through, because the flip-flop state was different at that point. That is to say, the Windows 3.1 standard mode grabber read from port 3C0h to query the attribute controller index register state, but the emulation returned the currently selected data register contents instead.

And then, when restoring the attribute controller index register the next time around, the register would be restored to the wrong value which didn't have bit 5 set, causing the screen to go blank.

Michal Necasek

It's not every day that you learn how an aspect of the workings of VGA causes a blank screen under very specific circumstances when running Windows 3.1 in Standard mode under emulation, and that this specific aspect of the workings of VGA was implemented to maintain backwards compatibility with EGA.

Absolutely bonkers.