LibVT: Keep track of the 'true' line endings

Author: https://github.com/alimpfard
Commit: https://github.com/SerenityOS/serenity/commit/424965954f8
Pull-request: https://github.com/SerenityOS/serenity/pull/8142
Issue: https://github.com/SerenityOS/serenity/issues/287
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibVT/Line.h

@@ -35,6 +35,7 @@ public:
 
     void clear(const Attribute& attribute = Attribute())
     {
+        m_terminated_at.clear();
         clear_range(0, m_cells.size() - 1, attribute);
     }
     void clear_range(size_t first_column, size_t last_column, const Attribute& attribute = Attribute());
@@ -50,15 +51,26 @@ public:
 
     void set_code_point(size_t index, u32 code_point)
     {
+        if (m_terminated_at.has_value()) {
+            if (index > *m_terminated_at) {
+                m_terminated_at = index + 1;
+            }
+        }
+
         m_cells[index].code_point = code_point;
     }
 
     bool is_dirty() const { return m_dirty; }
     void set_dirty(bool b) { m_dirty = b; }
 
+    Optional<u16> termination_column() const { return m_terminated_at; }
+    void set_terminated(u16 column) { m_terminated_at = column; }
+
 private:
     Vector<Cell> m_cells;
     bool m_dirty { false };
+    // Note: The alignment is 8, so this member lives in the padding (that already existed before it was introduced)
+    [[no_unique_address]] Optional<u16> m_terminated_at;
 };
 
 }

Userland/Libraries/LibVT/Terminal.cpp

@@ -9,6 +9,7 @@
 #include <AK/Debug.h>
 #include <AK/StringBuilder.h>
 #include <AK/StringView.h>
+#include <AK/TemporaryChange.h>
 #include <LibVT/Color.h>
 #include <LibVT/Terminal.h>
 #ifdef KERNEL
@@ -706,6 +707,10 @@ void Terminal::DCH(Parameters params)
 void Terminal::linefeed()
 {
     u16 new_row = cursor_row();
+#ifndef KERNEL
+    if (!m_controls_are_logically_generated)
+        active_buffer()[new_row].set_terminated(m_column_before_carriage_return.value_or(cursor_column()));
+#endif
     if (cursor_row() == m_scroll_region_bottom) {
         scroll_up();
     } else {
@@ -719,6 +724,7 @@ void Terminal::linefeed()
 void Terminal::carriage_return()
 {
     dbgln_if(TERMINAL_DEBUG, "Carriage return");
+    m_column_before_carriage_return = cursor_column();
     set_cursor(cursor_row(), 0);
 }
 
@@ -967,6 +973,7 @@ void Terminal::emit_code_point(u32 code_point)
     }
     if (m_stomp) {
         m_stomp = false;
+        TemporaryChange change { m_controls_are_logically_generated, true };
         carriage_return();
         linefeed();
         put_character_at(cursor_row(), cursor_column(), code_point);
@@ -980,6 +987,11 @@ void Terminal::emit_code_point(u32 code_point)
 
 void Terminal::execute_control_code(u8 code)
 {
+    ArmedScopeGuard clear_position_before_cr {
+        [&] {
+            m_column_before_carriage_return.clear();
+        }
+    };
     switch (code) {
     case '\a':
         m_client.beep();
@@ -1002,10 +1014,13 @@ void Terminal::execute_control_code(u8 code)
     case '\n':
     case '\v':
     case '\f':
+        if (m_column_before_carriage_return == m_columns - 1)
+            m_column_before_carriage_return = m_columns;
         linefeed();
         return;
     case '\r':
         carriage_return();
+        clear_position_before_cr.disarm();
         return;
     default:
         unimplemented_control_code(code);

Userland/Libraries/LibVT/Terminal.h

@@ -432,6 +432,9 @@ protected:
     Vector<bool> m_horizontal_tabs;
     u32 m_last_code_point { 0 };
     size_t m_max_history_lines { 1024 };
+
+    Optional<u16> m_column_before_carriage_return;
+    bool m_controls_are_logically_generated { false };
 };
 
 }

Userland/Libraries/LibVT/TerminalWidget.cpp

@@ -339,6 +339,11 @@ void TerminalWidget::paint_event(GUI::PaintEvent& event)
             if ((!visual_beep_active && !has_only_one_background_color) || should_reverse_fill_for_cursor_or_selection)
                 painter.clear_rect(cell_rect, terminal_color_to_rgb(should_reverse_fill_for_cursor_or_selection ? attribute.effective_foreground_color() : attribute.effective_background_color()));
 
+            if constexpr (TERMINAL_DEBUG) {
+                if (line.termination_column() == column)
+                    painter.clear_rect(cell_rect, Gfx::Color::Magenta);
+            }
+
             enum class UnderlineStyle {
                 None,
                 Dotted,