Coalesce the current line into a single chunk when moving away from it.

Editor/Editor.cpp

@@ -172,6 +172,7 @@ void Editor::move_down()
 {
     if (m_cursor.line() >= max_line())
         return;
+    coalesce_current_line();
     m_cursor.move_by(1, 0);
     if (m_cursor.column() > max_column())
         m_cursor.set_column(max_column());
@@ -179,10 +180,16 @@ void Editor::move_down()
     update_scroll_position_if_needed();
 }
 
+void Editor::coalesce_current_line()
+{
+    m_document->lines()[m_cursor.line()].coalesce();
+}
+
 void Editor::move_up()
 {
     if (m_cursor.line() == 0)
         return;
+    coalesce_current_line();
     m_cursor.move_by(-1, 0);
     if (m_cursor.column() > max_column())
         m_cursor.set_column(max_column());

Editor/Editor.h

@@ -56,6 +56,7 @@ private:
     void insert_at_cursor(int ch);
 
     void exec_command();
+    void coalesce_current_line();
 
     OwnPtr<Document> m_document;
 

Editor/Line.cpp

@@ -94,3 +94,13 @@ std::tuple<size_t, size_t> Line::chunk_index_for_position(size_t position)
     ASSERT(false);
     return std::make_tuple(0, 0);
 }
+
+void Line::coalesce()
+{
+    if (m_chunks.size() <= 1)
+        return;
+
+    auto contents = data();
+    m_chunks.clear();
+    m_chunks.push_back(Chunk{ contents });
+}

Editor/Line.h

@@ -30,6 +30,8 @@ public:
 
     void insert(size_t index, const std::string&);
 
+    void coalesce();
+
 private:
     void append(const std::string&);
     void prepend(const std::string&);