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

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);
 }
 

src/display_buffer.hh

@@ -24,11 +24,11 @@ struct DisplayAtom : public UseMemoryDomain<MemoryDomain::Display>
 public:
     enum Type { Range, ReplacedRange, Text };
 
-    DisplayAtom(const Buffer& buffer, BufferCoord begin, BufferCoord end, Face face = {})
-        : face(face), m_type(Range), m_buffer(&buffer), m_range{begin, end} {}
+    DisplayAtom(const Buffer& buffer, BufferRange range, Face face = {})
+        : face(face), m_type(Range), m_buffer(&buffer), m_range{range} {}
 
-    DisplayAtom(const Buffer& buffer, BufferCoord begin, BufferCoord end, String str, Face face = {})
-        : face(face), m_type(ReplacedRange), m_buffer(&buffer), m_range{begin, end}, m_text{std::move(str)} {}
+    DisplayAtom(const Buffer& buffer, BufferRange range, String str, Face face = {})
+        : face(face), m_type(ReplacedRange), m_buffer(&buffer), m_range{range}, m_text{std::move(str)} {}
 
     DisplayAtom(String str, Face face)
         : face(face), m_type(Text), m_text(std::move(str)) {}

src/highlighters.cc

@@ -747,7 +747,7 @@ struct WrapHighlighter : Highlighter
                     new_line.insert(new_line.begin(), {m_marker, face_marker});
                 if (indent > marker_len)
                 {
-                    auto it = new_line.insert(new_line.begin() + (marker_len > 0), {buffer, coord, coord});
+                    auto it = new_line.insert(new_line.begin() + (marker_len > 0), {buffer, {coord, coord}});
                     it->replace(String{' ', indent - marker_len});
                 }
 

src/window.cc

@@ -150,7 +150,7 @@ const DisplayBuffer& Window::update_display_buffer(const Context& context)
         LineCount buffer_line = setup.first_line + line;
         if (buffer_line >= buffer().line_count())
             break;
-        lines.emplace_back(AtomList{{buffer(), buffer_line, {buffer_line, buffer()[buffer_line].length()}}});
+        lines.emplace_back(AtomList{{buffer(), {buffer_line, {buffer_line, buffer()[buffer_line].length()}}}});
     }
 
     m_display_buffer.compute_range();

test/regression/4753-assert-in-display-line-split/cmd

@@ -0,0 +1 @@
+ypp

test/regression/4753-assert-in-display-line-split/in

@@ -0,0 +1 @@
+🔎

test/regression/4753-assert-in-display-line-split/rc

@@ -0,0 +1 @@
+add-highlighter global/ column 5 +r