Seems that primary selection is only half-supported. You can paste out (i.e. WebKit does update the selection clipboard when you select text) but you cannot paste into WebKit using middle-click. I thought we had a #PLATFORM(WAYLAND) check somewhere to turn off primary selection on Wayland, because it used to not be supported at all. I thought that might be the problem, but I can't find it anymore. Maybe we got rid of it.
Created attachment 302009 [details] Patch
Comment on attachment 302009 [details] Patch Clearing flags on attachment: 302009 Committed r212605: <http://trac.webkit.org/changeset/212605>
All reviewed patches have been landed. Closing bug.