[Zax]

Interesting trick! Thank you Kanegasi.

I tried this, but the result is sometime not very accurate:

-- params
local borderColor = {1, 1, 1}
local borderWidth = 1
--
if (not frame.SetBackdrop) then Mixin(frame, BackdropTemplateMixin) end
frame.backdrop = {
	edgeFile = "Interface\\Buttons\\WHITE8x8",
	tileEdge = false,
	edgeSize = borderWidth,
	insets = {left = borderWidth, right = borderWidth, top = borderWidth, bottom = borderWidth},
}
frame:SetBackdrop(frame.backdrop)
frame:SetBackdropBorderColor(borderColor[1], borderColor[2], borderColor[3], 1)