[pazz]

SimpleTree computed its root once, in the constructor. A tree built from an empty list therefore stayed empty after nodes were appended to that list: root stayed None and positions() yielded nothing. TreeListWalker also kept its own copy of the tree's root, and fixed its initial focus to the root at construction, so a TreeBox opened on an empty tree showed nothing even after the tree was filled and refreshed.

• SimpleTree.root is now a property: (0,) while the list has nodes, None while it is empty. Assigning root still works and overrides this (assigning None restores the default), as DecoratedTree.root (Read DecoratedTree.root from the wrapped tree #76) forwards assignments to the wrapped tree and its test assigns a SimpleTree's root.
• TreeListWalker.root reads the tree's root, and assigning it writes through to the tree, as DecoratedTree.root does since Read DecoratedTree.root from the wrapped tree #76. Before, assigning it changed only the walker's unused copy.
• A walker created without a focus follows the tree's root until a focus is set.

Tests: test_root_follows_list (append to and clear the list) and test_follows_root_of_tree_filled_later (a TreeBox on an empty tree, filled and refreshed). Both fail without the fix.

Review findings and resolution

review-correctness

• TreeListWalker.root became read-only: fixed, it now has a setter that writes through to the tree.
• SimpleTree.root = None cannot force an empty root: left, as an empty root on a non-empty list has no use; the docstring now says that assigning None restores the default.
• A walker without a focus follows a root that changes later: left, that is the intent of the change.
• The commit message does not say that a TreeBox must be refreshed: left, refresh() is the existing contract for any change to the tree.

review-design

• Use self.root in get_focus: fixed.
• State the read-only walker root and the None assignment in the description: done above (the walker root is no longer read-only).
• Nothing blocking.

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