With the latest update I now have 4K 60Hz using USB-C DP on my Dell XPS! (Up from 30hz on previous kernels/DTBs) So I actually now can use my external monitor properly! Great to see the progress.
I do have weird blue screens on my laptop screen sometimes when I disconnect the USB-C. Only a hard reset of the laptop works then. Maybe if I have time I’ll lookup some logs to see if the driver is reporting some issue, and report it upstream.
Also, the only way to get my wanted setup (only external monitor, and laptop screen off) is by booting laptop to Gnome, then select ‘Suspend’ in the power menu, immediately close the laptop screen, then immediately connect the USB-C monitor, and then 50% of the time I do get my monitor running, and the laptop screen is then not active anymore, so I don’t go ‘offscreen’ from my monitor to my laptop area. Can’t find a way to just disable it in Gnome or auto disable the laptop screen when closing lid. But hey, this is progress, cause now I can at least start setup my workflow instead of using my desktop Ubuntu machine!