As far as I can tell, no Firefox code uses it. But it's been in the toolkit forever. There's no documentation on MDN about it, either. It might be the least supported widget (and that's saying something)... Really, we should see if it's a candidate for removal as dead code. Or we should support it with documentation and a mochitest.

I still don't see any usage either, so I believe it can be removed: https://searchfox.org/mozilla-central/source/toolkit/content/xul.css#1159 https://searchfox.org/mozilla-central/source/toolkit/content/widgets/expander.xml https://searchfox.org/mozilla-central/search?q=expander

Comment on attachment 8931855 [details] [diff] [review] Removed expander binding and expander.xml You should also remove expander.css

Comment on attachment 8931855 [details] [diff] [review] Removed expander binding and expander.xml See above comment.

86ecce74, are you still working on this?

Pushed by dgottwald@mozilla.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/be780bf2c08a Removed expander binding, expander.css and expander.xml; r=enn