Created attachment 360336 [details]
WebkitWebView in magnifier
AcceleratedBackingStoreWayland uses gdk_cairo_draw_from_gl() for drawing on GTK+ 3.16.0+. For some reason, in GtkInspector's magnifier or when drawing into a non-default Cairo surface, it doesn't draw anything and generates GL_INVALID_OPERATION (when I manually add glGetError() checks, by default it doesn't print anything.
See attached screenshot
Fedora 29, Intel GPU, Wayland, org.gnome.Platform//master in Flatpak.
Forgot to mention that it specifically only happens with the GTK 3.16.0+ codepath which is a single call to gdk_cairo_draw_from_gl(). If I comment out that part to let it use the custom pipeline, it works just fine.
Created attachment 360573 [details]
Thanks for the patch. If this patch contains new public API please make sure it follows the guidelines for new WebKit2 GTK+ API. See http://trac.webkit.org/wiki/WebKitGTK/AddingNewWebKit2API
Comment on attachment 360573 [details]
I bet Miguel would want to review it before landing.
Committed r240712: <https://trac.webkit.org/changeset/240712>