[kurapica.igas]

It may cause taint, you don't need to replace the frame's meta, it is better to creating a wrapper of the frame like :

local metaF = {
    __index = {
        foo = function() print("bar") end
    },
}
 
function BuildFrame(...)
    local f = CreateFrame("Frame", ...)
 
    -- Only create the meta once
    if not metaF.Inited then
        metaF.Inited = true
        for k, v in pairs(getmetatable(f).__index) do
            if not metaF.__index[k] then
                metaF.__index[k] = v
            end
        end
    end
 
    -- Create the wrapper
    local w = setmetatable({}, metaF)
    w[0] = f[0]   -- Copy the userdata, make sure the widget API can works well with the wrapper
    -- w.RealUI = f   -- You may add a ref to the real frame
 
    return w
end
 
-- Now you can use it without taint
local frm = BuildFrame()
frm:SetBackdropColor(0,0.5,1)