[Krainz]

local addon = CreateFrame("Frame");
addon:RegisterEvent("PLAYER_LOGIN");
addon:RegisterEvent("PLAYER_REGEN_DISABLED");
addon:RegisterEvent("PLAYER_REGEN_ENABLED");
addon:RegisterEvent("PLAYER_STARTED_MOVING");
addon:RegisterEvent("PLAYER_STOPPED_MOVING");
 
 
addon:SetScript("OnEvent", function(self, event, ...)
 
    if event == "PLAYER_LOGIN"
    or event == "PLAYER_REGEN_DISABLED"
    or event == "PLAYER_STOPPED_MOVING" then
        
        if not UnitAffectingCombat("player") then
            UIFrameFadeOut(UIParent, 3, UIParent:GetAlpha(), 0);
        end;
    else
        UIParent:SetAlpha(1);
    end;
    
    if UnitAffectingCombat("player") then
        UIParent:SetAlpha(1);
    end;
end);
 
addon:SetScript("OnUpdate", function(self, elapsed)
 
    local speed = GetUnitSpeed("player");
    if speed ~=0 then
        UIParent:SetAlpha(1);
    else
        if GetMouseFocus() then
            if GetMouseFocus():GetName() ~= "WorldFrame" then
                UIParent:SetAlpha(1);
            else
                if not UnitAffectingCombat("player") then
                    UIFrameFadeOut(UIParent, 3, UIParent:GetAlpha(), 0.1);
                end;
            end;
        end;
    end;
    
    if UnitName("mouseover") then
        UIParent:SetAlpha(1);
    end;
end);

I tried to workaround the fade-in problem by changing it to SetAlpha(1).

It let to lots of clunky interactions, with the UI fading down and then showing up and then disappearing again, only to be hard to make it appear again and such. Lots of suboptimal interactions.