HDMI switcher not working on Mac

I'm deliberately doing a weird connection from my 15" Macbook Pro (recent touch bar model) to my dual external monitors on my desktop.

I have a situation where I have to swap three different computers into my workflow at different times: The Mac, my Windows laptop, and the Windows laptop from work.

Most of the time I just use my DisplayLink USB dock, and that's fine and work with the DisplayLink USB drivers in all three cases. Plug the DisplayLink dock it into the USB port on any of those three laptops, it works, I get working dual external displays for all three computers.

But there's a problem doing it that way, on the Mac only: When I run games, they give an error saying "OpenGL software mode not supported" and run super slow (1 fps) when the game window is on one of the external monitors running through the DisplayLink USB driver. When I drag the game window off of the external display down on to the real laptop screen, it runs fine at a normal frame rate.

So to work around this, have the Apple Mac USB-C Digital AV Multiport Adapter plugged in to the Mac (this is required since they don't have HDMI ports on Macs any more) and do a cable-swap on one of the external displays so that I'm now driving one of the external displays directly off the adapter's HDMI output, instead of doing it through the DisplayLink USB drivers. This works, and as long as the games are either on the monitor being direct-driven or on the laptop screen, they run at normal frame rates.

But I don't like swapping cables, so I tried getting an HDMI switch to leave plugged in. And it doesn't work, but only doesn't work when I'm using that Digital AV Multiport adapter. Symptom is: Mac doesn't detect an external display is connected at all.

Here are the things I tested:

Windows laptop -> HDMI Out -> HDMI switch -> External display = Works fine.

Mac Laptop -> Digital AV Multiport Adapter -> HDMI Out -> External display = Works fine.

Mac Laptop -> Digital AV Multiport Adapter -> HDMI Out -> HDMI Switch -> External display = Does not work.

I did all of the above with the laptop being the ONLY thing plugged into the HDMI switch input, so that I knew it wasn't switching confusion.

I tried two different brands of HDMI switches, same issue:
https://www.amazon.com/gp/product/B01N32XOIO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B06XTQ5N59/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

Anyone know what's going on here?

Interesting. If I use this Anker adapter for my HDMI adapter instead of the Mac Digital AV adapter, then everything works the way I expect it to work. It drives the HDMI switch the way I expect it to, and works correctly and displays a good image.

So this is a problem with the Mac adapter product.

What I love about this BBS is that if somebody asks a technical question, you're pretty much guaranteed that it's a non-trivial problem to fix.

So it seems to me that the hypothetical product that you want is a USB-C KVM switch. Of course, such a thing doesn't exist yet. What does seem to exist are DisplayPort and HDMI KVM switches. (Here's a are several multi-port multi-monitor HDMI models, of course, not exactly cheap...)

As an alternative, you could probably play games with screen-scraping / remote-desktop software (e.g., VNC). Not really useful for gaming, but perfectly useful for managing a fleet of test gear or whatnot.

Not exactly what I needed in this situation, but I understand why it would look like that, since my setup is really convoluted. :-)