Prevent the rectangle from being drawn continually, only at each new bar.

Yes, so that’s what I talked about. In history your calculation is made only once because the bars are already closed. While on the current bar, until its closure, your calculation is made on each tick and on each new tick you are increasing the column number (counter in your code?) and there creating X rectangles for the same bar and they are all painted until the bar is closed.

I have trouble making the rectangles to be detected and plotted with your version, what is instrument and timeframe please?

In order for your counter to increase only a new bar, change this function like this: (checking if timestamp of the bar is newer):

// Condition for rectangle
if  (b[1]=1) then
BeginnRechteckLong = 1
EndeRechteckLong = 4
Counter = LastSet($RechteckStartBar) + 1

ObereSeite = MeanderHI
UntereSeite = Middle1

// Saves the coordinates of the rectangle in array variables
$RechteckStartBar[Counter] = barindex - 1 + BeginnRechteckLong
$RechteckEndBar[Counter] = barindex - 1 + EndeRechteckLong
$RechteckObereSeite[Counter] = ObereSeite
$RechteckUntereSeite[Counter] = UntereSeite

// Updating the counter only if current bar has changed! 
if $lastbar[0]<opentimestamp then 
Counter = Counter + 1
$lastbar[0]=opentimestamp
endif 
endif