So I went ahead and wiped out ClutterContent from StTextureCache. Then wiped it out entirely from GNOME Shell.

I guess you mean you wiped out ClutterTexture?

Yup

Interestingly, XWayland windows are X11 windows with a Wayland surface. Which now makes it possible to find out if a particular window is an XWayland window using GNOME Shell's Looking Glass:

This was already possible by looking at MetaWindoWayland vs MetaWindowXwayland in a Wayland session. In an Xorg session, everything is MetaWindowX11.

MetaWindowActor has now the same split

It's not the same really. The MetaSurfaceActor split is for the backing (Pixmap vs wl_surface), while the MetaWindowActor split is for the window management (xdg-shell/wl_subcompositor et al vs X11 window manager magic).

Cogl uses color-based picking

Clutter uses ..