Anyway, many rabbit holes later, I found out that Teams uses an undocumented hard-coded call to the dictionary used by Google Chrome. You cannot edit the dictionary, you cannot supplement it with a custom dictionary, and you cannot modify the hard-code address or use a different dictionary. You can't even turn off this call.
And don't get me started on how Teams ignores every UI standard that other Windows programs use, and it's basically not modifiable in any way.
Thanks, I hate it.
I grimaced when we started using Slack back in 2016: those ~200ms UI delays between switching conversation tabs (and the ~3s freezes when switching accounts) were incredibly grating - but we hoped they’d fix the worst user-facing parts of Electron (and the excessive RAM usage) but 7 years later it’s still as bad as it’s always been.
What’s their excuse?