Re: Issue 3330: redo much of the stencil stacking/spacing/empty-check (issue 8869044)

[k-ohara5a5a]

You are struggling because the code recursively combines the tail of a
sequence of stencils, before adding the first.

Presumably we want the first line to overstrike, and the second to
  \markup{ "A" \hspace #-2.5 "--- the end" }
  \markup{ "A" \center-column {"up" "down"}}
but both attach the "A" to tails with the same shape
  [.A] [   .    ]

One way to distinguish the cases is to remember a cursor position, the
extent-start of the most-recently-added element in the tail, rather than
use the extent-start of the combined tail.  (More easily thought of in a
loop implementation, rather than recursion.)

https://codereview.appspot.com/8869044/