A folder whose children should appear only when the folder
is selected MUST place those children in a different window
than the folder itself (typically WINDOW toolbar). In a
horizontal toolbar such as System.root, a folder that keeps
its children in its own window cannot switch anything — they
are shown flattened next to it and selecting the folder does
nothing. A vertical toolbar instead renders same-window
children as a nested group under the folder, so there the
separate window is not required.