Fix splitting of display atoms accross multi-columns codepoint

Honor the split request by inserting an empty atom to make sure
client code can assume splitting does replace one atom with two

Fixes #4753

1 files changed, 4 insertions, 0 deletions

diff --git a/src/display_buffer.cc b/src/display_buffer.cc
index 335fb5de..c9b74d04 100644
--- a/src/display_buffer.cc
+++ b/src/display_buffer.cc
@@ -108,6 +108,10 @@ DisplayLine::iterator DisplayLine::split(iterator it, ColumnCount count)
     auto pos = utf8::advance(get_iterator(it->buffer(), it->begin()),
                              get_iterator(it->buffer(), it->end()),
                              count).coord();
+    if (pos == it->begin()) // Can happen if we try to split in the middle of a multi-column codepoint
+        return m_atoms.insert(it, {it->buffer(), {pos, pos}, it->face});
+    if (pos == it->end())
+        return std::prev(m_atoms.insert(std::next(it), {it->buffer(), {pos, pos}, it->face}));
     return split(it, pos);
 }