document tree: [i0 -> [x-foo]]
x-foo's shadow tree: [j1 -> [s1] -> [s2] -> j2 ->[x-bar]]
x-bar's shadow tree: [k1 -> [s3]]
slot #s1: [i1 -> i2]
slot #s2: [i3]
slot #s3: [[s4]]
slot #s4: [i4 -> i5]

v1 ideal nav forward: [i0 -> j1 -> i1 -> i2 -> i3 -> j2 -> x-bar -> k1 -> i4 -> i5]