[Ammako]

I'm going to go with a much cleaner, less error-prone solution, actually.

local function setAlphaZero()
    if not CompactRaidFrameManagerDisplayFrame:IsVisible() then
        CompactRaidFrameManager:SetAlpha(0)
    end
end
 
CompactRaidFrameContainer:SetParent(UIParent)
CompactRaidFrameManager:SetAlpha(0)
 
CompactRaidFrameManager:HookScript("OnEnter", function() CompactRaidFrameManager:SetAlpha(1) end)
CompactRaidFrameManager:HookScript("OnLeave", setAlphaZero)
 
CompactRaidFrameManagerToggleButton:HookScript("OnEnter", function() CompactRaidFrameManager:SetAlpha(1) end)
CompactRaidFrameManagerToggleButton:HookScript("OnLeave", setAlphaZero)

I tried replacing the hooks on ToggleButton with an extra CompactRaidFrameManager:IsMouseOver() check, like what Fizzle did, but it gave wonky results where the side bar remains visible when you close the raid frame manager drawer, until you manually move your mouse back over and away again to "refresh" it, so I'll probably keep it like that.