[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
            C_Timer.After(3, 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
                C_Timer.After(3, UIFrameFadeOut(UIParent, 3, UIParent:GetAlpha(), 0))
                end;
            end;
        end;
    end;
    
    if UnitName("mouseover") then
        UIParent:SetAlpha(1);
    end;
end);

I also tried to work around the "timer" solution through C_Timer, but then I realized that the interaction was already happening after out of combat, but just delayed. The user must STAY idle for that amount of time in order for the interaction to be optimal.

It ended up working like a delayed time bomb. Very, very clunky.