From fe58d15b5b860b060b5ef477095e5cd65ee84cf5 Mon Sep 17 00:00:00 2001
From: Daniel Bertalan <dani@danielbertalan.dev>
Date: Sat, 5 Jun 2021 07:23:01 +0200
Subject: [PATCH] LibVT: Fix `ECH` not clearing lines

The `num` parameter should be treated as an offset from the cursor
position, not from the beginning of the line. The previous behavior
caused fragments of previous lines to be visible when moving the entire
buffer in vim (e.g. with `gg` and `G`).

The debug messages I used while fixing it are also included in this
commit. These will help diagnose further issues if they arise.
---
 Userland/Libraries/LibVT/Terminal.cpp | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/Userland/Libraries/LibVT/Terminal.cpp b/Userland/Libraries/LibVT/Terminal.cpp
index 413d572c90..1323bdac83 100644
--- a/Userland/Libraries/LibVT/Terminal.cpp
+++ b/Userland/Libraries/LibVT/Terminal.cpp
@@ -30,6 +30,7 @@ Terminal::Terminal(Kernel::VirtualConsole& client)
 #ifndef KERNEL
 void Terminal::clear()
 {
+    dbgln_if(TERMINAL_DEBUG, "Clear the entire screen");
     for (size_t i = 0; i < rows(); ++i)
         active_buffer()[i].clear(m_current_state.attribute);
     set_cursor(0, 0);
@@ -557,8 +558,10 @@ void Terminal::ECH(Parameters params)
     unsigned num = 1;
     if (params.size() >= 1 && params[0] != 0)
         num = params[0];
-    // Clear from cursor to end of line.
-    for (unsigned i = cursor_column(); i < num; ++i) {
+    // Clear num characters from the right of the cursor.
+    auto clear_end = min<unsigned>(m_columns, cursor_column() + num - 1);
+    dbgln_if(TERMINAL_DEBUG, "Erase characters {}-{} on line {}", cursor_column(), clear_end, cursor_row());
+    for (unsigned i = cursor_column(); i <= clear_end; ++i) {
         put_character_at(cursor_row(), i, ' ');
     }
 }
@@ -570,19 +573,19 @@ void Terminal::EL(Parameters params)
         mode = params[0];
     switch (mode) {
     case 0:
-        // Clear from cursor to end of line.
+        dbgln_if(TERMINAL_DEBUG, "Clear line {} from cursor column ({}) to the end", cursor_row(), cursor_column());
         for (int i = cursor_column(); i < m_columns; ++i) {
             put_character_at(cursor_row(), i, ' ');
         }
         break;
     case 1:
-        // Clear from cursor to beginning of line.
+        dbgln_if(TERMINAL_DEBUG, "Clear line {} from the start to cursor column ({})", cursor_row(), cursor_column());
         for (int i = 0; i <= cursor_column(); ++i) {
             put_character_at(cursor_row(), i, ' ');
         }
         break;
     case 2:
-        // Clear the complete line
+        dbgln_if(TERMINAL_DEBUG, "Clear line {} completely", cursor_row());
         for (int i = 0; i < m_columns; ++i) {
             put_character_at(cursor_row(), i, ' ');
         }
@@ -600,7 +603,7 @@ void Terminal::ED(Parameters params)
         mode = params[0];
     switch (mode) {
     case 0:
-        // Clear from cursor to end of screen.
+        dbgln_if(TERMINAL_DEBUG, "Clear from cursor ({},{}) to end of screen", cursor_row(), cursor_column());
         for (int i = cursor_column(); i < m_columns; ++i)
             put_character_at(cursor_row(), i, ' ');
         for (int row = cursor_row() + 1; row < m_rows; ++row) {
@@ -610,7 +613,7 @@ void Terminal::ED(Parameters params)
         }
         break;
     case 1:
-        // Clear from cursor to beginning of screen.
+        dbgln_if(TERMINAL_DEBUG, "Clear from beginning of screen to cursor ({},{})", cursor_row(), cursor_column());
         for (int i = cursor_column(); i >= 0; --i)
             put_character_at(cursor_row(), i, ' ');
         for (int row = cursor_row() - 1; row >= 0; --row) {