LibVT: Update the terminal buffer based on visible lines

Lines in the history should not be considered for update at all.
Fixes #2185

Libraries/LibVT/Terminal.h

@@ -144,6 +144,15 @@ public:
         return m_lines[index - m_history.size()];
     }
 
+    Line& visible_line(size_t index)
+    {
+        return m_lines[index];
+    }
+    const Line& visible_line(size_t index) const
+    {
+        return m_lines[index];
+    }
+
     size_t max_history_size() const { return 500; }
     const NonnullOwnPtrVector<Line>& history() const { return m_history; }
 

Libraries/LibVT/TerminalWidget.cpp

@@ -465,9 +465,9 @@ void TerminalWidget::flush_dirty_lines()
     }
     Gfx::Rect rect;
     for (int i = 0; i < m_terminal.rows(); ++i) {
-        if (m_terminal.line(i).dirty) {
+        if (m_terminal.visible_line(i).dirty) {
             rect = rect.united(row_rect(i));
-            m_terminal.line(i).dirty = false;
+            m_terminal.visible_line(i).dirty = false;
         }
     }
     update(rect);