When using the Outline view instead of TreeTableView, I do not see expansion sign for rows with children.
The row can be expanded, thus some active component that expands it is likely there, but it's not visible.
On others L&F it's visible.
reassigning to new component owner
fixed in core-main/rev/ff5762238fa9
Integrated into 'main-golden', will be available in build *200812131401* on http://bits.netbeans.org/dev/nightly/ (upload may still be in progress)
User: Jiri Rechtacek <email@example.com>
Log: #150115: No expansion sign is visible on GTK L&F in Outline view