Function lazy_stack_axis 

pub fn lazy_stack_axis(layout: &dyn Layout) -> Option<LazyStackAxis>

Resolves a layout to the axis a lazy container can virtualize along.

Every renderer that virtualizes a lazy container asks this, so the answer is the same everywhere. Deriving it per backend is how GTK ended up reading Layout::stretch_axis — a different question entirely, and one whose answer changed when the stacks became content-sized, silently laying every lazy horizontal stack out vertically.