diff --git a/src/display_buffer.cc b/src/display_buffer.cc
index 4a8233ba..fc5b7bb7 100644
--- a/src/display_buffer.cc
+++ b/src/display_buffer.cc
@@ -79,17 +79,36 @@ DisplayBuffer::DisplayBuffer()
 {
 }
 
+DisplayBuffer::iterator DisplayBuffer::insert(iterator where, const DisplayAtom& atom)
+{
+    check_invariant();
+    iterator res = m_atoms.insert(where, atom);
+    check_invariant();
+    return res;
+}
+
+DisplayBuffer::iterator DisplayBuffer::atom_containing(const BufferIterator& where)
+{
+    for (iterator it = m_atoms.begin(); it != m_atoms.end(); ++it)
+    {
+        if (it->end() > where)
+            return it;
+    }
+    return end();
+}
+
 DisplayBuffer::iterator DisplayBuffer::split(iterator atom, const BufferIterator& pos)
 {
     assert(atom < end());
     assert(pos > atom->begin());
     assert(pos < atom->end());
+    check_invariant();
     DisplayAtom new_atom(atom->coord(), atom->begin(), pos,
                          atom->fg_color(), atom->bg_color(), atom->attribute());
 
     atom->m_begin = pos;
     atom->m_coord = new_atom.end_coord();
-    iterator res = insert(atom, std::move(new_atom));
+    iterator res = m_atoms.insert(atom, std::move(new_atom));
     check_invariant();
     return res;
 }
diff --git a/src/display_buffer.hh b/src/display_buffer.hh
index 874276a8..d4f69b23 100644
--- a/src/display_buffer.hh
+++ b/src/display_buffer.hh
@@ -99,7 +99,7 @@ public:
 
     void clear() { m_atoms.clear(); }
     void append(const DisplayAtom& atom) { m_atoms.push_back(atom); }
-    iterator insert(iterator where, const DisplayAtom& atom) { return m_atoms.insert(where, atom); }
+    iterator insert(iterator where, const DisplayAtom& atom);
     iterator split(iterator atom, const BufferIterator& pos);
 
     void replace_atom_content(iterator atom, const BufferString& replacement);
@@ -110,6 +110,8 @@ public:
     const_iterator begin() const { return m_atoms.begin(); }
     const_iterator end()   const { return m_atoms.end(); }
 
+    iterator atom_containing(const BufferIterator& where);
+
     const DisplayAtom& front() const { return m_atoms.front(); }
     const DisplayAtom& back()  const { return m_atoms.back(); }