[sduguay-devolutions]

Issue

With ExperimentalOffscreen on, video and audio never play. The page loads fine and
the element is there, it just never starts. The same page plays on the non-offscreen
adapter.

The offscreen adapter parents the web view in a GtkOffscreenWindow, and WebKit won't
start media when the toplevel is one. That decision is made inside WebKit from the
type of the toplevel.

Fix

Use a window that draws offscreen without being a GtkOffscreenWindow.
GtkOffscreenHostWindow registers a GObject type at runtime deriving from GtkWindow,
and in class_init copies the class-struct slots where GtkOffscreenWindow differs from
GtkWindow into its own class struct. Rendering is the same as before, but WebKit sees a
plain GtkWindow toplevel and media plays.

Before (nothing load):

After:
Screencast_20261013_164149.webm