I assume garbling windows isn't technically mandated by the spec, it's just what DWM does when Windows tells it to "rearrange" all windows on a desktop that has lost a bunch of displays, because Windows helpfully noticed that they were turned off.
https://www.reddit.com/r/nvidia/comments/66opvp/this_display... https://answers.microsoft.com/en-us/windows/forum/all/how-to... https://answers.microsoft.com/en-us/windows/forum/windows_7-... (AMD workaround) https://www.overclock.net/forum/74-graphics-cards-general/12... https://superuser.com/questions/630555/turning-displayport-m... https://www.nvidia.com/en-us/geforce/forums/discover/209725/... https://www.reddit.com/r/Windows10/comments/4895u9/turning_o... https://www.reddit.com/r/nvidia/comments/9qn0dk/has_anyone_f... https://www.reddit.com/r/windows/comments/788l4f/put_one_dis... and many, many more, like this one: https://what.thedailywtf.com/topic/24548/displayport-is-the-...