[SDPhantom]

Strange, that's the approach I tried the first time, but it didn't work out for me. I get the feeling this mechanic has more events firing that they aren't exposing to Lua side. Anyway, this is the code I have. It's really robust in which I like to have anything I can think of covered.

This also allows any addon wanting to poll GarrisonMissionFrame.MissionComplete.completeMissions get updated info.

Note: nop() is a global dummy function in UIParent.lua

LoadAddOn("Blizzard_GarrisonUI");
GarrisonMissionFrame_CheckCompleteMissions=nop;
 
local EFrame=CreateFrame("Frame");
EFrame:RegisterEvent("GARRISON_MISSION_NPC_OPENED");--      Open Command Table
EFrame:RegisterEvent("GARRISON_MISSION_NPC_CLOSED");--      Close Command Table
EFrame:RegisterEvent("GARRISON_MISSION_FINISHED");--        Mission Completed
EFrame:RegisterEvent("GARRISON_MISSION_COMPLETE_RESPONSE");--   Mission Turned In
 
local MissionsOpen=false;
local function ScanMissions(delay)
    if not MissionsOpen then return; end
    if not delay then
        local list=C_Garrison.GetCompleteMissions();
        GarrisonMissionFrame.MissionComplete.completeMissions=list;
        if #list<=0 then GarrisonMissionList_UpdateMissions(); return; end
 
        for i,j in ipairs(list) do
            C_Garrison.MarkMissionComplete(j.missionID);
        end
    end
    C_Timer.After(0.5,ScanMissions);
end
 
EFrame:SetScript("OnEvent",function(self,event,...)
    if event=="GARRISON_MISSION_NPC_OPENED" then
        MissionsOpen=true;
        ScanMissions();
    elseif event=="GARRISON_MISSION_NPC_CLOSED" then
        MissionsOpen=false;
    elseif event=="GARRISON_MISSION_FINISHED" and MissionsOpen then
        C_Garrison.MarkMissionComplete((...));
        ScanMissions(true);
    elseif event=="GARRISON_MISSION_COMPLETE_RESPONSE" then
        if C_Garrison.CanOpenMissionChest((...)) then C_Garrison.MissionBonusRoll((...)); end
    end
end);