Made detailed nUnit tests for all the methods of the TreeVew (LoadTree, ObjectTreeList, ClearTreeList, SelectNode, OnNodeMouseClick, OnTreeMouseLeave, OnTreeMouseMove) also checking for GDI leaks and didn't find any problems. Improved also the tests for PropertyGid and also no leaks found.
In Naro the TreeView seems to leak, reimplemented the mouse hover code to remove flickering (caused by the fact that the tree is completely redrawn) and it seems that also the leaking disappeared. Need some more functionality checking before committing.
Will continue investigating the GDI leaking caused by the the OCC visual window.