I’ve recently updated to the latest v3.4.14028 64-bit pre-release from another several-months-old pre-release version.
'i' should “show page information in full screen/presentation mode”, as the manual puts it. This used to work very nicely in all previous Sumatra versions I’ve used. However, since the recent update, pressing
'i' inverts PDF color (exchanging black and white). See example in the below image.