Gets vertical skylines from grob stencils

input/regression/outside-staff-placement-directive.ly

+\version "2.17.0"
+
+\header {
+  texidoc = "@code{VerticalAxisGroup} grobs can place outside staff objects
+using one of the four directives shown below.
+"
+}
+
+
+\layout {
+  ragged-right = ##t
+  indent = 0.0
+  \context {
+    \Voice
+    \remove "Ligature_bracket_engraver"
+    \consists "Mensural_ligature_engraver"
+  }
+  \context {
+    \Score
+    \override SpacingSpanner #'packed-spacing = ##t
+    \override PaperColumn #'keep-inside-line = ##f
+  }
+}
+
+music = \context Voice {
+  \clef "petrucci-c4"
+  \set Staff.printKeyCancellation = ##f
+  \cadenzaOn % turn off bar lines
+  \accidentalStyle "forget"
+  \textLengthOn
+
+% ligaturae binaria
+
+  \[
+    b\breve^\markup { \column { { \bold "ligaturae binaria" } "BL" } }
+    g\longa
+    \]
+
+  \[
+    g\breve^\markup { "BL" }
+    b\longa
+    \]
+
+  \[
+    b\longa^\markup { "LL" }
+    g
+    \]
+
+  \[
+    g\longa^\markup { "LL" }
+    b
+    \]
+
+  \[
+    b\breve^\markup { "BB" }
+    g
+    \]
+
+  \[
+    g\breve^\markup { "BB" }
+    b
+    \]
+
+  \[
+    b\longa^\markup { "LB" }
+    g\breve
+    \]
+
+  \[
+    g\longa^\markup { "LB" }
+    b\breve
+    \]
+
+  \[
+    b1^\markup { "SS" }
+    g
+    \]
+
+  \[
+    g1^\markup { "SS" }
+    b
+    \]
+
+  \bar "|"
+}
+
+{
+  \override Staff.VerticalAxisGroup #'outside-staff-placement-directive =
+    #'left-to-right-polite
+  \music
+}
+{
+  \override Staff.VerticalAxisGroup #'outside-staff-placement-directive =
+    #'left-to-right-greedy
+  \music
+}
+{
+  \override Staff.VerticalAxisGroup #'outside-staff-placement-directive =
+    #'right-to-left-polite
+  \music
+}
+{
+  \override Staff.VerticalAxisGroup #'outside-staff-placement-directive =
+    #'right-to-left-greedy
+  \music
+}