title: TextBlock issue when long text and TextTrimming
body: <p>I have a TextBlock with <code>TextTrimming="CharacterEllipsis"</code>, <code>TextWrapping="NoWrap"</code> and very long text about 140 000 symbols. I put this text into the TextBlock, and then the TextBlock shows this text as multiline control whith trimming in an each row.</p>
  
  <h2>XAML</h2>
  
  <pre><code>    &lt;Window x:Class="WpfApplication3.MainWindow"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              Title="MainWindow" Height="350" Width="525"&gt;
          &lt;Grid&gt;
              &lt;TextBlock x:Name="textBlock" TextTrimming="CharacterEllipsis" TextWrapping="NoWrap" Width="500"/&gt;
          &lt;/Grid&gt;
      &lt;/Window&gt;
  </code></pre>
  
  <h2>C#</h2>
  
  <pre><code>    public partial class MainWindow : Window {
              public MainWindow() {
                  InitializeComponent();
                  string str = "q ";
                  for(int i = 0; i &lt; 20000; i++) {
                      str += "qwetre ";
                  }
                  textBlock.Text = str;
              }
          }
  </code></pre>
  
  <p><br />
  <img src="https://i.stack.imgur.com/XR8LW.png" alt=""></p>
  
  <p>Any ideas? Is it bug of MS, or I can change this behaviour?</p>