Hmm, it's probably due to how nUI was initially written ( and beyond my skills to understand where the problem is ) but ...
EditMode start and finish announces an error when I have nUI active but not when I have my minimal test actionbar addon active.
Hopefully other UI replacement addons ( especially older ones ) won't have the same problem but here is the error message I received.
Lua Code:
2x [ADDON_ACTION_FORBIDDEN] AddOn 'nUI' tried to call the protected function 'TargetUnit()'.
[string "@Interface/AddOns/!BugGrabber/BugGrabber.lua"]:519: in function <Interface/AddOns/!BugGrabber/BugGrabber.lua:519>
[string "=[C]"]: in function `TargetUnit'
[string "@Interface/FrameXML/EditModeManager.lua"]:1414: in function `RefreshTargetAndFocus'
[string "@Interface/FrameXML/EditModeManager.lua"]:1354: in function `OnEditModeEnter'
[string "@Interface/FrameXML/EditModeManager.lua"]:162: in function `EnterEditMode'
[string "@Interface/FrameXML/EditModeManager.lua"]:183: in function <Interface/FrameXML/EditModeManager.lua:181>
[string "=[C]"]: in function `Show'
[string "@Interface/FrameXML/UIParent.lua"]:2871: in function `SetUIPanel'
[string "@Interface/FrameXML/UIParent.lua"]:2677: in function `ShowUIPanel'
[string "@Interface/FrameXML/UIParent.lua"]:2581: in function <Interface/FrameXML/UIParent.lua:2577>
[string "=[C]"]: in function `SetAttribute'
[string "@Interface/FrameXML/UIParent.lua"]:3174: in function `ShowUIPanel'
[string "*:OnClick"]:2: in function <[string "*:OnClick"]:1>