How to create a rectangle in Pine Script showing the range of the first 90 minutes?

I am using a 5-minute chart. I'd like to draw a transparent rectangle from bar no. 19 of the session all the way to the final bar of the session, showing the range of the first 90 minutes (ie, the first 18 bars in a 5 minute chart).

//@version=5
indicator("18-bar range", overlay = true)

var float rangeHigh = 0
var float rangeLow = 0
box myBox = na

newDay = ta.change(time("D"))
barsCount = ta.barssince(newDay)

if (barsCount == 0)
    rangeHigh := high
    rangeLow := low

if (barsCount < 18)
    if (high > rangeHigh)
        rangeHigh := high
    if (low < rangeLow)
        rangeLow := low

if (barsCount >= 18)
    myBox := box.new(left = bar_index + 1, top = rangeHigh, right = bar_index, bottom = rangeLow, border_color = color.new(#cce2cc, 90), bgcolor = color.new(#cce2cc, 50))

The problem I have is that the rectangle for some weird reason is starting from bar 28, not bar 19 as intended. What am I doing wrong?

This is due to the default number of boxes that Pine can draw. The way your code is set up you are drawing a new box on every new bar past barsCount 18. The maximum default number of boxes is 50. You can fix this two ways.

First using the max_boxes_count in your indicator function.

indicator("18-bar range", overlay = true, max_boxes_count = 500)

Second way you can set your box variable using var var box myBox = na and then the set_right this will only have one box per day and just continue to extend the right with every new bar

if (barsCount == 18)
    myBox := box.new(left = bar_index + 1, top = rangeHigh, right = bar_index, bottom = rangeLow, border_color = color.new(#cce2cc, 90), bgcolor = color.new(#cce2cc, 50))
if barsCount >= 18
    myBox.set_right(bar_index)