MacBook and iPad: External display uncontrollable in Sidecar with Universal Control

I am not in distress and this is definitely a "Steak is too juicy and lobster too buttery" kind of problem. I am continually astounded that this hardware could all work together so seamlessly and this is a bit of an interesting conundrum.

• In this instance: a MacBook Neo and iPad Pro M4.
• Using two external 4K monitors, Universal Control and Sidecar.
• Both devices are set to use Extended Desktop rather than Mirror.
• Four individual desktop spaces to manage, through some method (touch, trackpad, etc.)

These illustrations show the challenge. The arrangement shows the order of the displays, in virtual space, and the path the cursor follows in Universal Control, running from screen-to-screen.

In the first illustration, the MacBook has no problem controlling all four displays. Obviously the most-right display is not a touch screen so you need the Mac's trackpad to drive it. Nonetheless, it gives up nothing else. It's another iPad space and functions just as the main iPad display does.

Wanting to find the limits of all of this: "What happens if I put the iPad into Sidecar mode?" Would it extend both of the iPad screens onto the Mac?

The result of using things like the second slide is the issue. The last screen on the right stays in iPad mode. The iPad main display goes into Sidecar mode and works fine.

The screen at the far right becomes control-orphaned. It's still there. You can see it. There's a dock at the bottom of the window.

• I just can't control it when the iPad is in Sidecar mode.
• Exit Sidecar mode and everything goes back to normal.

Does anyone have any ideas how to get control of that last display? Either as an iPad with Universal Control, or extending the Mac desktop to the fourth device?

Why? Who cares?

I have a PC sitting to my right, for old engineering programs, that is going to get switched on less and less. There's a nice 32" display with an extra HDMI input that goes unused. Connecting the iPad to that display seems obvious. I'm trying to figure out all the ways it can be used.

I thought this might be an interesting discussion. It might also give some people ideas of how to better use their own devices. I discovered most of these capabilities by accident.