Tree Traversal is unlike the Mercury Native

Tree Traversal is unlike the Mercury Native
 
Steps to reproduce:
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1)Run SwingSet2, with the gtktheme file as the vm argument.
2)Change the look and feel to GTK.
3) Go to Tree Demo.
4) Select any node and press the right arrow key.
5) The traversal continues till the last node of the tree.
6) Now Open Configuration Editor on Mercury OS and repeat step 4. This is unlike the Java implementation.
This was tested against Tiger b29 and on Mercury build10