Mysterious behaviour with UserControl and DocumentList during desi

I wrote a simple control derived from UserControl which draws a diagonal
line. When I drag this control to a form, the control is displayed, the
diagonal line is shown in the form designer. When I add a second form to this
solution and put a DocumentList control on that second form, the UserControl
on the first form doesn't show the line any more during design time. All
design time behaviour is gone. Removing the DocumentList control from the
second form, all works well.

What am I missing?