Fix horizontal scrolling support with replaced buffer ranges

tab character were not properly handled when scrolling horizontally

src/display_buffer.cc

@@ -5,6 +5,24 @@
 namespace Kakoune
 {
 
+void AtomContent::trim_begin(CharCount count)
+{
+    if (m_type == BufferRange)
+        m_begin = utf8::advance(m_buffer->iterator_at(m_begin),
+                                m_buffer->iterator_at(m_end), count).coord();
+    else
+        m_text = m_text.substr(count);
+}
+
+void AtomContent::trim_end(CharCount count)
+{
+    if (m_type == BufferRange)
+        m_end = utf8::advance(m_buffer->iterator_at(m_end),
+                              m_buffer->iterator_at(m_begin), -count).coord();
+    else
+        m_text = m_text.substr(0, m_text.char_length() - count);
+}
+
 DisplayLine::iterator DisplayLine::split(iterator it, BufferCoord pos)
 {
     kak_assert(it->content.type() == AtomContent::BufferRange);
@@ -65,6 +83,37 @@ CharCount DisplayLine::length() const
     return len;
 }
 
+void DisplayLine::trim(CharCount first_char, CharCount char_count)
+{
+    for (auto it = begin(); first_char > 0 and it != end(); )
+    {
+        if (not it->content.has_buffer_range())
+        {
+            ++it;
+            continue;
+        }
+
+        auto len = it->content.length();
+        if (len <= first_char)
+        {
+            m_atoms.erase(it);
+            first_char -= len;
+        }
+        else
+        {
+            it->content.trim_begin(first_char);
+            first_char = 0;
+        }
+    }
+    auto it = begin();
+    for (; it != end() and char_count > 0; ++it)
+        char_count -= it->content.length();
+
+    if (char_count < 0)
+        (it-1)->content.trim_end(-char_count);
+    m_atoms.erase(it, end());
+}
+
 void DisplayBuffer::compute_range()
 {
     m_range.first  = {INT_MAX,INT_MAX};

src/display_buffer.hh

@@ -97,6 +97,8 @@ public:
 
     Type type() const { return m_type; }
 
+    void trim_begin(CharCount count);
+    void trim_end(CharCount count);
 private:
     friend class DisplayLine;
 
@@ -152,6 +154,10 @@ public:
     iterator insert(iterator it, DisplayAtom atom) { return m_atoms.insert(it, std::move(atom)); }
     void     push_back(DisplayAtom atom) { m_atoms.push_back(std::move(atom)); }
 
+    // remove first_char from the begining of the line, and make sure
+    // the line is less that char_count character
+    void trim(CharCount first_char, CharCount char_count);
+
     void     optimize();
 private:
     LineCount m_buffer_line;

src/window.cc

@@ -69,17 +69,17 @@ void Window::update_display_buffer()
         LineCount buffer_line = m_position.line + line;
         if (buffer_line >= buffer().line_count())
             break;
-        const String& content = buffer()[buffer_line];
-        BufferCoord begin{buffer_line, content.byte_count_to(m_position.column)};
-        BufferCoord end = buffer().advance(buffer_line, content.byte_count_to(m_position.column + m_dimensions.column));
-
         lines.push_back(DisplayLine(buffer_line));
-        lines.back().push_back(DisplayAtom(AtomContent(buffer(), begin, end)));
+        lines.back().push_back(DisplayAtom(AtomContent(buffer(), buffer_line, buffer_line+1)));
     }
 
     m_display_buffer.compute_range();
     m_highlighters(*this, m_display_buffer);
     m_builtin_highlighters(*this, m_display_buffer);
+
+    // cut the start of the line before m_position.column
+    for (auto& line : lines)
+        line.trim(m_position.column, m_dimensions.column);
     m_display_buffer.optimize();
 
     m_timestamp = buffer().timestamp();
@@ -152,12 +152,12 @@ void Window::scroll_to_keep_cursor_visible_ifn()
             if (first_col < m_position.column)
                 m_position.column = first_col;
             else if (column >= m_position.column + m_dimensions.column)
-                m_position.column = column - (m_dimensions.column - 1);
+                m_position.column = column - m_dimensions.column;
 
             if (last_col < m_position.column)
                 m_position.column = last_col;
             else if (column >= m_position.column + m_dimensions.column)
-                m_position.column = column - (m_dimensions.column - 1);
+                m_position.column = column - m_dimensions.column;
 
             return;
         }