Windows 3.x VDDVGA

While working on my Windows 3.x display driver, I ran into a vexing problem. In Windows 3.1 running in Enhanced 386 mode, I could start a DOS session and switch it to a window. But an attempt to set a mode in the DOS window (e.g. MODE CO80) would destroy the Windows desktop, preventing further drawing from happening properly. It was possible to recover by using Alt+Enter to switch the DOS window to full screen again and then returning to the desktop, but obviously that wasn't going to cut it.

Oddly enough, this problem did not exist in Windows 3.0. And in fact it also didn't exist in Windows 3.1 if I used the Windows 3.0 compatible VDDVGA30.386 VxD shipped with Windows 3.1 (plus the corresponding VGA30.3GR grabber).

A classic retro computing bug hunt! These stories are always a good read.