Running Epiphany 3.18.0 and Webkit2Gtk 2.10.1 under gnome-shell & libmutter 3.18.1, I cannot copy/paste into or out of form controls between processes (including WebKitWebProcess instances). E.g. Copying text from gedit and pasting into a textarea (and vice-versa) fails, and copying from a textarea in one tab and pasting into another also fails. The clipboard only seems to work when copying/pasting within the same WebKitWebProcess instance.
Are you running GNOME in Wayland or X11?
Ah, title of the bug says WAYLAND. I think this was never implemented; if it was working for you in 2.8, it was because of XWayland. (Wayland support is new in 2.10.)
Oh yeah, that might not have been too obvious, sorry. Didn't try with 2.8 - I was just running a GNOME3 Wayland session for a few days to see if it was worth switching to permanently yet.
This seems to be a duplicate of bug 146574...
Oh, yes - sure is a dupe. *** This bug has been marked as a duplicate of bug 146574 ***