True, but it's exactly what I've been doing for years (since Black Morass in TBC) and I've never had anything not work as a result of tainting WSAUF's SetPoint. Though, I'll grant that it's probably not something you should do in a public library.
1. ChocolateBar tells LibJostle "I'm putting a bar here, move this frame out of the way".
2. LibJostle moves the frame out of the way.
3. The default UI moves the frame back.
4. LibJostle moves the frame out of the way again.
5. Repeat #3 and #4 over and over.
I'd think it would be simpler to just overwrite the frame's SetPoint method with an empty function, so the default UI could call it all it wanted without having any effect, than trying to move it back and forth repeatedly.
Right, but Jostle doesn't do anything on its own... I guess my question should have been "why is ChocolateBar asking Jostle to move stuff in combat?" Every bar addon I've used just moves things out of the way once each time a new bar is added to the screen (including when you log in), so unless you're adding new bars in the middle of a fight...
Well, DBM could also do something about it on its end, by not parenting its secure frames to the WorldStateAlwaysUpFrame. Instead, it could simply get the position of the WSAUF, and anchor its secure frames to the UIParent in the same location. It could also post-hook SetPoint on the WSAUF to ensure that its buttons moved if the frame did.
Though, I'm not really sure why LibJostle is trying to move the frame in combat anyway. Most addons I've seen that use LibJostle just move stuff out of the way once at login, or if a new bar is created, or if you change the width of the bar, none of which are likely to be happening while you're in combat. Sounds like ChocolateBar is doing something weird, too.
Nobody is saying DBM is doing something wrong, per se. Re-read my post. I said that the problem could be caused by the combination of one addon that adds secure stuff to the world state frame for battlegrounds, and another addon that tries to move the world state frame without checking to see if another addon has caused it to be protected. And, since it was purely hypothesizing on my part, the problem could even be caused by something totally unrelated.
Well, the other point to mention is that WorldStateAlwaysUp frame isn't protected, so the error is probably caused by some addon adding "click to target the flag carrier" buttons, or something along those lines, and then another addon (such as ChocolateBar, through Jostle) trying to do something that would normally be okay but can't be done with a protected frame.
Searching for "WorldStateAlwaysUpFrame" won't necessarily locate the problem, either, since addons that add stuff to the frame usually attach their extra parts to its children instead. Try searching just for "AlwaysUp".
Out of your addon list, I'd guess that DeadlyBossMods is the most likely culprit, as it does have battleground-related features and likely touches the WorldStateAlwaysUpFrame. Try turning off any PvP-related features and seeing if the problem goes away. After that, CT_Mod would be my next guess... is the CT stuff even actively developed anymore? I can't remember the last time I heard anyone mention it.
You'd probably get more results if you posted a list of addons you're using.
Also, rather than disabling addons one by one, you could speed up the process considerably. If you actually want to play battlegrounds while you're testing, start by disabling everything you don't need to do that; if the problem exists in that group, then you've already narrowed it down quite a bit, but if it doesn't, then you can continue by enabling half of the rest of your addons. If that half doesn't trigger the errror, disable it, and enable the other half. Once you find the half with the problem, disable half of those, etc. Continue until you narrow it down to a single addon. It's still a pain, but it's way faster than the "one by one" approach unless you only have a few addons.