[TBC and upwards]

Hello, you need to use "FocusGuid"

Sorry, that is what I meant. I use that, but it does not set focus correctly, it seems.

Hmmm, it seems to work fine, but for some of my users, it bugs out. At first I thought it was this, then I thought it was maybe SpellManager.CastSpellByNameOn, but I tried it all and it all works.

I think than Wow addon can cause this problem. You can also try to lock frame (to be sure than focus don't change)