TreeV3 folder nodes don't retain isFolder state when modified

[guest]

A tree node that is marked with isFolder="true" does not retain the folder state if it becomes empty.

The bug can be reproduced using tests/widget/treeV3/dnd1.html

• Run the dnd1.html test
• Drag and drop a node onto the "Empty Folder" node, so that it's no longer empty.
• Drag the node that was just added to "Empty Folder" out, so that "Empty Folder" is emtpy again.
• Notice that "Empty Folder" now shows as a leaf node, rather than a folder. It appears as if isFolder="false".

[ilia]

That's a feature, settable in TreeV3 properties.