Thanks for this tip - I didn’t know that was a quick way to get the task bar!
But I don’t think ALT TAB itself was the cause - switching apps with MSFS under full load was the clue (it didn’t happen when MSFS was on lower settings…).
After a lot of googling, the answer may have been in this forum already - many people have reported that sometimes Windows will switch MSFS from the GPU to the iGPU, if one exists on your CPU, as a misguided power saving measure. I assume the logic is something like this: when you put MSFS in the background to bring another app to the foreground, when the GPU is saturated, Windows tries to offload MSFS to the iGPU and put it in power saving. this would explain MSFS going to single digit FPS and effectively locking up without any sign of an actual crash… It’s also probably because I have the default Windows “Balanced” power plan, and the default “Normal” NVIDIA Control Panel power management (I want the GPU to be able to idle when it wants).
The solution - if it is indeed the reason - is to go in to Settings > System > Displays > Graphics, and explicitly assign MSFS to the GPU, so Windows knows not to use iGPU.
I don’t know for sure, but so far I have not had the issue occur again. I also don’t know if this affects other things.
