SharedTooltipBackdropCompatibility.lua

function SharedTooltip_InitBackdropForTooltip(tooltip)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    Mixin(nineSlice, BackdropTemplateMixin);
    nineSlice:SetScript("OnSizeChanged", nineSlice.OnSizeChanged);

    nineSlice.backdropInfo = tooltip.backdropInfo;
    nineSlice.backdropColor = tooltip.backdropColor;
    nineSlice.backdropColorAlpha = tooltip.backdropColorAlpha;
    nineSlice.backdropBorderColor = tooltip.backdropBorderColor;
    nineSlice.backdropBorderColorAlpha = tooltip.backdropBorderColorAlpha;
    nineSlice.backdropBorderBlendMode = tooltip.backdropBorderBlendMode;

    nineSlice:OnBackdropLoaded();
end

function SharedTooltip_GetNineSliceFrame(tooltip)
    return tooltip.NineSlice or tooltip;
end

function SharedTooltip_ApplyBackdrop(tooltip)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    nineSlice:ApplyBackdrop();
end

function SharedTooltip_GetBackdrop(tooltip)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    return nineSlice:GetBackdrop();
end

function SharedTooltip_SetBackdrop(tooltip, backdropInfo)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    nineSlice:SetBackdrop(backdropInfo);
end

function SharedTooltip_GetBackdropColor(tooltip)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    return nineSlice:GetBackdropColor();
end

function SharedTooltip_SetBackdropColor(tooltip, r, g, b, a)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    nineSlice:SetBackdropColor(r, g, b, a);
end

function SharedTooltip_GetBackdropBorderColor(tooltip)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    return nineSlice:GetBackdropBorderColor();
end

function SharedTooltip_SetBackdropBorderColor(tooltip, r, g, b, a)
    local nineSlice = SharedTooltip_GetNineSliceFrame(tooltip);
    nineSlice:SetBackdropBorderColor(r, g, b, a);
end

---

local TestTooltip = CreateFrame("GameTooltip", "TestTooltip", UIParent, "GameTooltipTemplate");
GameTooltip_SetDefaultAnchor(TestTooltip, UIParent);
TestTooltip:SetMinimumWidth(128, true);
TestTooltip:SetText("Hello!", 1, 1, 1, nil, true);

SharedTooltip_InitBackdropForTooltip(TestTooltip);
SharedTooltip_SetBackdrop(TestTooltip,
    {
        bgFile = [[Interface\Buttons\WHITE8X8]],
        edgeFile = [[Interface\Buttons\WHITE8X8]],
        tile = true,
        tileEdge = true,
        tileSize = 8,
        edgeSize = 8,
    }
);
SharedTooltip_SetBackdropColor(TestTooltip, 1, 0, 0, 0.5);
SharedTooltip_SetBackdropBorderColor(TestTooltip, 0, 1, 0, 0.5);

TestTooltip:Show();