[BUG] Cursor off by 1 when running inside tmux (Alacritty + Windows)

Description

Cursor position is off by 1 character when running Claude Code inside tmux on Windows. The misalignment only occurs with Claude Code — normal shell usage within the same tmux + Alacritty setup has correct cursor positioning.

Environment

• OS: Windows 11 Pro (25H2) x86_64
• Terminal: Alacritty
• Multiplexer: psmux (tmux wrapper for PowerShell on Windows)
• Font: Hack Nerd Font Mono
• tmux default-terminal: tmux-256color
• tmux allow-passthrough: on

Steps to reproduce

1. Open Alacritty on Windows
2. Start a tmux session via psmux
3. Launch claude inside the tmux pane
4. Type text — cursor is offset by 1 character from its expected position

What was tried (none resolved the issue)

• Changed default-terminal from xterm-256color to tmux-256color
• Set allow-passthrough off (reverted, no effect)
• Added terminal-overrides for Alacritty (RGB, Tc)
• Set unicode-ambiguous-as-wide on

Expected behavior

Cursor should align with the typed text, matching the behavior seen in normal shell usage within the same tmux session.

Additional context

Running Claude Code directly in Alacritty without tmux does not exhibit this issue. The problem is specific to the Claude Code TUI rendering inside tmux.