LibLine: Use the real shown line count around in cleanup()

Previously we would leave artifacts on screen if a change caused the
buffer to span fewer lines than the current buffer.
This commit records the shown line count and uses that instead of trying
to guess the previous line count (and failing most of the time).

Userland/Libraries/LibLine/Editor.cpp

@@ -1296,9 +1296,8 @@ void Editor::cleanup()
 {
     auto current_buffer_metrics = actual_rendered_string_metrics(buffer_view(), m_current_masks);
     auto new_lines = current_prompt_metrics().lines_with_addition(current_buffer_metrics, m_num_columns);
-    auto shown_lines = num_lines();
-    if (new_lines < shown_lines)
-        m_extra_forward_lines = max(shown_lines - new_lines, m_extra_forward_lines);
+    if (new_lines < m_shown_lines)
+        m_extra_forward_lines = max(m_shown_lines - new_lines, m_extra_forward_lines);
 
     OutputFileStream stderr_stream { stderr };
     reposition_cursor(stderr_stream, true);
@@ -1313,6 +1312,8 @@ void Editor::refresh_display()
     DuplexMemoryStream output_stream;
     ScopeGuard flush_stream {
         [&] {
+            m_shown_lines = current_prompt_metrics().lines_with_addition(m_cached_buffer_metrics, m_num_columns);
+
             auto buffer = output_stream.copy_into_contiguous_buffer();
             if (buffer.is_empty())
                 return;

Userland/Libraries/LibLine/Editor.h

@@ -432,6 +432,7 @@ private:
     size_t m_num_lines { 1 };
     size_t m_previous_num_columns { 0 };
     size_t m_extra_forward_lines { 0 };
+    size_t m_shown_lines { 0 };
     StringMetrics m_cached_prompt_metrics;
     StringMetrics m_old_prompt_metrics;
     StringMetrics m_cached_buffer_metrics;