In Gimp 2.99, I wanted to make a fancy custom GUI for a python 3 plugin, but currently I am stuck. If I create a Gtk.Window and show it from the main thread of my plugin, the window contents are not drawn (The title text is rendered). I guess that this is a threading problem. So I tried putting my GTK code in a function called by GLib.idle_add(). But it seems that the function is never invoked. I replaced the GTK code with a few print and stderr write statements, and they never do anything. It is as if my function is the lowest priority, and is always skipped.
Oh, and BTW, In GIMP 2.10, functions invoked from GLib.idle_add() in Python 2.7 work as expected.
I am now going to try to skip GTK, and see what I can do with the GimpUi library. I hope it does not have similar issues.