[p3lim]

Try this out

local queued, casting
 
local OnEnter = function(self)
    if(not casting) then
        local castbar = YourPlayerFrame.Castbar
        castbar:ClearAllPoints()
        castbar:SetAllPoints(self.Health)
    else
        queued = self
    end
end
 
local OnHide = function()
    local castbar = YourPlayerFrame.Castbar
    castbar:ClearAllPoints()
    castbar:SetAllPoints(YourPlayerFrame.Health)
end
 
local CastStarted = function()
    casting = true
end
 
local CastEnded = function()
    if(queued) then
        OnEnter(queued)
        queued = nil
        casting = nil
    else
        OnHide()
    end
end
 
-- on the raid frames:
-- Return the castbar if the unit disappears.
self:HookScript('OnHide', OnHide)
self:HookScript('OnEnter', OnEnter)
 
-- on the player frame:
castbar.PostCastFailed = CastEnded
castbar.PostCastInterrupted = CastEnded
castbar.PostCastStop = CastEnded
castbar.PostChannelStop = CastEnded
castbar.PostCastStart = CastStarted
castbar.PostChannelStart = CastStarted