<p>I attached a picture where you can see the bug. </p> <p>This behaviour does occur if you do the following things in three pane mode:</p> <ol> <li>Read a <span style="font-weight: bold;">pure</span> html-e-mail (in preview mode) </li> <li>Go to the next, previous or any other message (still in preview mode) </li> <li>Now drag the panes to make them smaller or bigger</li> </ol> <p>The multiple borders have no negative side effect, but they don't look good. [H] </p>