Hi Cormac, that's not a bug: On Windows, the top resize border is inside of the window, while the other three resize borders are outside of the window. You only get a resize cursor if the area below the cursor is draggable, which your green HBox is not. If you make it draggable, then you'll also get a resize cursor. This is by design, as the resize border shouldn't overlap client elements (in our case, elements that are not draggable).
The fact that you get a resize cursor for the top edge of the red label looks like it may have something to do with the fact that it's a label, but in reality the label is just placed close enough to the top edge of the window so that the resize border extends a tiny bit over the label (which is draggable).
