[Terenna]

I modified it ever so slightly to this:

local GetBestMapForUnit = C_Map.GetBestMapForUnit   
local MapRects = {}
local mapID
local UnitPosition = UnitPosition
local GetWorldPosFromMapPos = C_Map.GetWorldPosFromMapPos
local CreateVector2D = CreateVector2D
 
local function GetPlayerMapPosition(mapid, token, rects, _, x, y)
    x, y = UnitPosition(token)
    if not x then return end
    if mapid then
        rects = MapRects[mapid]
        if not rects then
            rects = {}
            _, topleft = GetWorldPosFromMapPos(mapid, CreateVector2D(0,0))
            _, bottomright = GetWorldPosFromMapPos(mapid, CreateVector2D(1,1))
 
            bottomright:Subtract(topleft)
            rects = { topleft.x, topleft.y, bottomright.x, bottomright.y }
            MapRects[mapid] = rects
        end
        
        x, y = x - rects[1], y - rects[2]
        return y / rects[4], x / rects[3]
    end
end
 
local function OnUpdate(self, elapsed, x, y)
    self.elapsed = self.elapsed + elapsed
    while self.elapsed > 0.5 do --only update the coords 2/second
        if mapID then
            x, y = GetPlayerMapPosition(mapID, 'player')
            if x then
                self.text:SetFormattedText('%.1f, %.1f', x * 100, y * 100)
            else
                self.text:SetText('')
            end
        end
 
        self.elapsed = self.elapsed - 0.5
    end
end
 
tMinimapCoordsFrame:SetScript('OnEvent', function(self, event, ...)
    if event == 'PLAYER_STOPPED_MOVING' then
        mapID = nil
        tMinimapCoordsFrame:SetScript('OnUpdate', nil)
    elseif event == 'PLAYER_STARTED_MOVING' then
        mapID = GetBestMapForUnit('player')
        tMinimapCoordsFrame:SetScript('OnUpdate', OnUpdate)
    elseif event == 'ZONE_CHANGED_NEW_AREA' then
        if IsInInstance() then
            self:UnregisterEvent('PLAYER_STARTED_MOVING')
            self:UnregisterEvent('PLAYER_STOPPED_MOVING')
            self:UnregisterEvent('ZONE_CHANGED')
            self:UnregisterEvent('ZONE_CHANGED_INDOORS')
        else
            self:RegisterEvent('PLAYER_STOPPED_MOVING')
            self:RegisterEvent('PLAYER_STARTED_MOVING')
            self:RegisterEvent('ZONE_CHANGED')
            self:RegisterEvent('ZONE_CHANGED_INDOORS')
        end
    else
        mapID = GetBestMapForUnit('player')
        OnUpdate(self, 1) --force it to update the coords exactly once as we log in
        self:UnregisterEvent('PLAYER_ENTERING_WORLD')
        if IsInInstance() then
            self:UnregisterEvent('PLAYER_STARTED_MOVING')
            self:UnregisterEvent('PLAYER_STOPPED_MOVING')
            self:UnregisterEvent('ZONE_CHANGED')
            self:UnregisterEvent('ZONE_CHANGED_INDOORS')
        end
    end
end)

I reworked the OnEvent handler to unregister and register the movement and zone changes if the player is in an instance so it won't be running in there where there are no coordinates.