title: Why doesn't Flowdocument's tooltip show?
body: <p>I have a very strange problem with tooltips inside FlowDocument. Finally I am able to narrow it down to the following xaml. Paste the following xaml into kaxaml or Blend, you will see the problem.</p>
  
  <pre><code>&lt;FlowDocumentScrollViewer&gt;
      &lt;FlowDocument Background="Transparent"&gt;
          &lt;Table&gt;
              &lt;Table.Columns&gt;
                  &lt;TableColumn Width="15" /&gt;
                  &lt;TableColumn /&gt;
              &lt;/Table.Columns&gt;
              &lt;TableRowGroup&gt;
                  &lt;TableRow&gt;
                      &lt;TableCell&gt;
                          &lt;Paragraph Margin="0,3.10333333333333,0,0"&gt;
                              &lt;Run FontSize="5"&gt;●&lt;/Run&gt;
                          &lt;/Paragraph&gt;
                      &lt;/TableCell&gt;
                      &lt;TableCell&gt;
                          &lt;Paragraph&gt;
                              &lt;Run FontSize="13"
                                   ToolTip="This is a tooltip"&gt;I have a tooltip&lt;/Run&gt;
                          &lt;/Paragraph&gt;
                          &lt;Paragraph LineHeight="0.1"
                                     Background="Transparent"&gt;
                              &lt;Figure Name="MyFigure"
                                      HorizontalAnchor="ColumnLeft"
                                      VerticalAnchor="ParagraphTop"
                                      CanDelayPlacement="False"
                                      Width="200"
                                      Padding="0,0,0,0"&gt;
                                  &lt;BlockUIContainer&gt;
                                      &lt;Grid&gt;
                                          &lt;Rectangle Name="MyRectangle"
                                                     Fill="Green"
                                                     Width="Auto"
                                                     Height="50" /&gt;
                                      &lt;/Grid&gt;
                                  &lt;/BlockUIContainer&gt;
                              &lt;/Figure&gt;
                          &lt;/Paragraph&gt;
                      &lt;/TableCell&gt;
                  &lt;/TableRow&gt;
              &lt;/TableRowGroup&gt;
          &lt;/Table&gt;
      &lt;/FlowDocument&gt;
  &lt;/FlowDocumentScrollViewer&gt;
  </code></pre>
  
  <p>The tooltip doesn't open. However if I do one of the following, the tooltip will show.</p>
  
  <ul>
  <li>Change MyFigure's Width to 15</li>
  <li>Change MyRectangle's Height to 5</li>
  </ul>
  
  <p>It almost feels like the Run is covered by something so its tooltip doesn't show.</p>