The BlackBox created "surround/border" frames that was presumably to hide the television "noise" effect some older versions of WoW could get when you peeked behind the WorldFrame and didn't cover it with UI elements.
The last three lines in Core.lua do the real work of adjusting your view into the game.
WorldFrame now seems to be a "restricted" frame for anchoring other frames.
Code to create the error in 8.2:
Code:
local frame = CreateFrame("Frame")
frame:ClearAllPoints()
frame:SetPoint("TOPLEFT", WorldFrame)
frame:SetPoint("BOTTOMRIGHT", UIParent)