I know that docs say that the tree order determines the render layer, but this isn't always true. It's hard to articulate the mismatch I'm experiencing, so I whipped up this diagram:
I hope this makes sense. For my ordering/indexing I've just been using the tree order and haven't adjusted any z-indexes, and yet the visual ordering doesn't match the tree order. The yellow-marked "StatusBlock" tooltip renders below things it overlays in its tree, but render above the red-marked "UI" which overlays it in the parent tree.
From the docs:
Perhaps counterintuitively, the topmost nodes in the scene panel are drawn on behind lower ones in the viewport
I may have missed some strange behavior with tooltips or control elements or something, but everything I'm finding about this says to use tree order.
Maybe using a CanvasLayer would help, but then the "StatusBlock" element changes position according to viewport, not the Unit Node.