document tree: [i0 -> [x-foo]]
x-foo's shadow tree: [j1 -> j2 -> [s1]]
slot #s1: [k1 -> [x-bar] -> k0 -> [s2] -> [s3]]
x-bar's shadow tree: [m1 -> m2]
slot #s2: [i1 -> i2]
slot #s3: [l1]

v1 ideal nav forward: [i0 -> j1 -> j2 -> k1 -> x-bar -> m1 -> m2 -> k0 -> i1 -> i2 -> l1]