Well, after thinking about it for a few minutes, I realized that Popout Manager probably doesn’t need to see the windows to detect them. So I added the PFD and then used the Manager to pop it out, and it showed the missing PFD’s coordinates in it’s UI.
Setting it’s XPOS back to 0 from whatever strange value was in there got it back onto my main display, after which I could drag it back to the wired tablet and resize it normally.
Intererstingly, MultiMonitorTool sees the “Panel2 (Custom)” window for the MFD correctly on the display it’s shown on, but it doesn’t see the “Panel1 (Custom)” window for the PFD on the Splashtop Wired xDisplay, as it is. It shows it as still on my main display, though it very definitely isn’t.
If anything I would think it would be the other way around, as the MFD is on a fake-display created using the open-source Indirect Display Driver and shown via VNC, and the PFD is on the USB-connected tablet using the better-supported LuminonCore IDDCX driver.
I suspect what happened is that I re-arranged the relative positions of the displays in Windows while not in the cockpit. And stored positions of the popped out PFD and MFDs were relative to the main window anchor, and so sensitive to the Windows display arrangement. The sim happily pops them out to a relative X,Y location at which there is no longer a display–even though that same display is still connected.
If that’s the problem, theoretically solution is to close all popouts before re-arranging the secondary displays they are on. The problem is that would include all aircraft for which there is save popout position state.
Not sure whose bug this is, but these two tools together should help avoiding the need to reset your profile and/or cloud-save.
You can also try moving your connected displays around in the Windows “re-arrange” to see if you can ‘capture’ the popouts again, but it’s simple enough to reset their position after Popout Manger detects the window, wherever it is.