I've seen little about iPadOS 26 and external display support if I'm honest, even amongst all the 'ipads now replace Macs' hype. These discussions or videos often focus on the iPad on its own, then often conclude that actually they don't quite replace Macs for most people, but disregard one of...
EDIT: I apparently put a malformed image as a background image. Fix was relocating the ~/Library/Application Support/Dock folder to the desktop and logging out/in (or restarting). It reset the Dock and its contents but fixed the problem.
###
On the Mac App Store I bought 'uDesktop NEXT' which...