add BufferPane::scroll_window_up() and scroll_window_down()

src/gui/BufferPane.cc

@@ -608,6 +608,30 @@ void BufferPane::enter_insert_mode(Window::EnterInsertModeMode which)
     m_window->request_redraw();
 }
 
+void BufferPane::scroll_window_up()
+{
+    const int lines_to_scroll = 3;
+    int so = effective_scroll_offset();
+    int lines_to_move_cursor = (so + lines_to_scroll) - (m_rows - m_cursor_screen_row - 1);
+    while (lines_to_move_cursor-- > 0)
+    {
+        move_cursor_screen_row_up();
+    }
+    m_cursor_screen_row += lines_to_scroll;
+}
+
+void BufferPane::scroll_window_down()
+{
+    const int lines_to_scroll = 3;
+    int so = effective_scroll_offset();
+    int lines_to_move_cursor = (so + lines_to_scroll) - m_cursor_screen_row;
+    while (lines_to_move_cursor-- > 0)
+    {
+        move_cursor_screen_row_down();
+    }
+    m_cursor_screen_row -= lines_to_scroll;
+}
+
 void BufferPane::forward_to_column(int column, bool allow_eol)
 {
     Buffer::Iterator start_of_line = *m_iterator;

src/gui/BufferPane.h

@@ -21,6 +21,8 @@ public:
     void kill_character_at_cursor();
     void write_file();
     bool insert_mode() const { return m_buffer->insert_mode(); }
+    void scroll_window_up();
+    void scroll_window_down();
 
 protected:
     int effective_scroll_offset()