This is introduced from 8u20, but doesn't happen to 8, so it could be are regression.

Please run attached sample, when mouse single clicking on the arrow icons of the TreeTableView, in most of the time the tree is not expanded/collapsed, just the cell/row is selected. Only when mouse clicking in the red rectangle area(refer to the snapshot), the expand/collapse action works, so it only works for the root of the TreeTableView.

TreeView works fine.