The picker sorted folders A-Z by path, with Inbox first, so a folder dragged into place in the sidebar turned up somewhere else when moving mail. It now walks the tree in compareFolders order, the sidebar's order with every folder expanded: Inbox, then the saved order, then the special folders, then A-Z, with subfolders under their parent. treeOrder lives beside compareFolders. A folder the walk from the top cannot reach is appended rather than dropped, so it stays pickable as it was before. Closes #1 (cherry picked from commit ea03406646062359f74e16ad8a8aed074b4dc409)