I think we’ve got it pinned down for the most part. It seems to be related to some libdecor changes that were added recently to enable window decorations. Historically, the window has just floated in the air in wayland, but the libdecor stuff lets it act like a normal program window.
This PR fixes the 0 B window issue, apparently: https://github.com/libretro/RetroArch/pull/13759
tyvm. Just built it, from this latest merge ("twinaphex merged commit 065fb5a into libretro:master) and it opens! 
[INFO] RetroArch 1.10.1 (Git 065fb5acee)
[...]
[ERROR] [Wayland/Vulkan]: Failed to draw splash screen
[INFO] [Vulkan]: Vulkan dynamic library loaded.
[INFO] [Vulkan]: Found vulkan context: "vk_wayland".
[INFO] [Vulkan]: Detecting screen resolution: 1920x1080w00t, thanks for testing/verifying! That’s great news.
It’ll make it to the testing PPA soon, but I think we’re planning to do another stable this weekend so I’ll hold off on the stable until then.
can someone help me with this please