Index: trunk/src/org/openstreetmap/josm/gui/ExtendedDialog.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/ExtendedDialog.java	(revision 9865)
+++ trunk/src/org/openstreetmap/josm/gui/ExtendedDialog.java	(revision 9866)
@@ -435,10 +435,10 @@
     /**
      * Tries to find a good value of how large the dialog should be
-     * @return Dimension Size of the parent Component or 2/3 of screen size if not available
+     * @return Dimension Size of the parent component if visible or 2/3 of screen size if not available or hidden
      */
     protected Dimension findMaxDialogSize() {
         Dimension screenSize = GuiHelper.getScreenSize();
         Dimension x = new Dimension(screenSize.width*2/3, screenSize.height*2/3);
-        if (parent != null) {
+        if (parent != null && parent.isVisible()) {
             x = JOptionPane.getFrameForComponent(parent).getSize();
         }