Excel 2013 shape height change display mystery

I'm trying to understand this. I have a shape that I want to animate its height growing. Like this

Sub ShapeHeightAnimation()

    Dim Total As Double

    ActiveSheet.Shapes("Rectangle 1").Height = 0
        
    For i = 1 To 100
        ActiveSheet.Shapes("Rectangle 1").Height = i
        
        'slow down
        For iRow = 1 To 8000
            Total = Total + Sheet2.Cells(iRow, 1).Value / 1000
        Next iRow        
    Next i

End Sub

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BUT, the shape does not change height during the For-Next loop, but it updates the display only when the code has finished running. (Note the 'slow down')
I tried Application.ScreenUpdating = True but had no effect.

My simple animation test should work, right?