VT emulator: LF incorrectly resets cursor column to 0

[Bahtya]

Summary

TerminalScreen::linefeed() resets cursor.col = 0 in addition to advancing the row. Per VT100/ECMA-48 spec, LF (line feed, 0x0A) should only move the cursor down one row — it must NOT reset the column position. The column reset is the responsibility of CR (carriage return, 0x0D).

Impact

Programs that rely on LF-only cursor movement (without CR) will have incorrect cursor positioning. This affects:

• ncurses-based TUI applications that position the cursor then use LF to move down
• Any program that emits standalone LF characters for vertical cursor movement
• cmd.exe/PowerShell output that uses LF without CR in certain modes

Most Unix shells pair CR+LF (\r\n), so this bug is masked for basic command output. However, TUI programs and some Windows console applications may emit standalone LF.

Affected Code

crates/kestrel-tools/src/builtins/terminal/screen.rs:854-861:

fn linefeed(&mut self) {
    self.cursor.col = 0;  // BUG: should NOT reset column
    if self.cursor.row == self.scroll_bottom {
        self.scroll_up(1);
    } else if self.cursor.row < self.active_buf().rows - 1 {
        self.cursor.row += 1;
    }
}

Expected Behavior

fn linefeed(&mut self) {
    // Do NOT reset cursor.col
    if self.cursor.row == self.scroll_bottom {
        self.scroll_up(1);
    } else if self.cursor.row < self.active_buf().rows - 1 {
        self.cursor.row += 1;
    }
}

Note

Some terminal emulators implement an "automatic newline" mode (LNM, DECSET 20) where LF does imply CR. However, this mode is rarely enabled and should be opt-in, not the default behavior. The default should be LF-only (no column reset).