The title pretty much says it all. Paste a string longer than 9600 characters into a TextBox that has TextWrapping set to TextWrapping.NoWrap, and it will split it into 9600-character lines, wrap them, and grow vertically. Is there a good reason for this? Any way to prevent it?
Note that "you shouldn't use TextBox for a string that long" is a valid opinion, but doesn't answer the question. :)
The XAML to demonstrate this doesn't need to be any more complicated than:
<Window x:Class="TestApp.TestWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="TestWindow" Height="300" Width="300">
    <TextBox TextWrapping="NoWrap" HorizontalAlignment="Left" VerticalAlignment="Top" />
</Window>