Scenes that have no children displayed (either because there was nothing
to display or because we didn't expand the sub-tree, such as in the
"Unused Data" display mode) would always have a chevron icon indicating
the collapsed state, but it wasn't interactive. As such it just felt
broken. This explicit exception to always draw the chevron for scene
elements was added a long time ago (13 years ago or longer), and I
couldn't find any good rationale for it.
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