I've developed a VBA-style add-on for PowerPoint in C# using NetOffice Framework, https://github.com/NetOfficeFw . The add-on is now working, and in fact my question is not really related to that.
On my desktop PC I have four screens, and when PowerPoint is in "slide show mode" it takes over two of the screens, one for the slide show and one for the presenter screen, both shown in full screen mode. The normal PowerPoint window is also still there on one of the other two screens.
On my old portable PC with an extra screen connected, the extra screen normally shows a mirror image of the built-in display. But when PowerPoint goes into "slide show mode" it somehow reconfigures the system and shows the slide show in full screen on the connected screen and the presenter screen on the built-in display. Very clever.
But what exactly has PowerPoint done, and how? Is this documented anywhere?
This is mostly just to satisfy my curiosity. I have now recoded my add-on so it works - I was previously using the .Net System.Windows.Forms.Screen class to figure out where to position the mouse cursor when I'd placed PowerPoint in "slide show mode", but that doesn't work on my portable PC because it maintains that there is still only one screen on the system.
Older versions of PPT behave differently but current versions, when you start a slide show, change your windows video settings from mirror to extend and put the slide show view on the second monitor unless you override the default settings. Then it resets the video to the previous setting when you end the slide show.
When you extend the display across multiple monitors, Windows sees it as one large screen, which explains why your PC reports only the one screen, I think.