Factorio Forums

https://lua-api.factorio.com/latest/LuaGuiElement.html

"flow": An invisible container that lays out its children either horizontally or vertically. The root GUI elements (top, left, center and screen; see LuaGui) are flows.

"empty-widget": An empty element that just exists. The root GUI element screen is an empty-widget.

I doubt that the screen root GUI is both a flow and an empty-widget. (And if it would be, it should be mentioned somewhere that it actually is both and how that would work).

Thank you for the report. top, left, center and goal are indeed flows, while screen and relative are empty-widgets. The docs have been updated to reflect this for the next release.