Oops! Thank you. 
Interesting though; the Glimmer stuff should be compatible with 3.5. So the upstream port to Glimmer may be immature. I guess I’ll try it out, and maybe look at it on 3.6. Thanks.
Update: false alarm, more 3.6-specific changes added but the 3.5 version fixes the warnings about removing widgets. So I’m happy.
PS moving the buttons (my change) fixes an upstream bug (category not auto-detected). See here: