For some reason, I'm not seeing UNIT_SPELLCAST_SENT firing for me (or more likely I'm not bound to it somehow).
I tried building a new frame from the console in game and registering on that (shamelessly stolen from a post by haste a few years ago) and that does catch the event as expected:
Lua Code:
f = CreateFrame'Frame' f:RegisterEvent("UNIT_SPELLCAST_SENT") f:SetScript("OnEvent", function(...) print(...) end)
You can see the code here:
Github, but the general idea is that it's calling oUF:RegisterEvent and passing a callback function. I'm doing this with other events successfully.
Any thoughts or areas to look at would be greatly appreciated. Thanks!