Windows: Backspace in $EDITOR=vim.exe launched via Ctrl+G inserts ⌂ (0x7F) instead of deleting

[DLehenbauer]

Repro

• Windows 11, PowerShell (conhost or Windows Terminal)
• Copilot CLI v1.0.49-1
• Vim 9.2 (console build) at C:\Program Files\Vim\vim92\vim.exe

$Env:EDITOR = "$Env:ProgramFiles\Vim\vim92\vim.exe"
copilot
# Press Ctrl+G (or Ctrl+X then e) to edit the prompt in $EDITOR
# In Vim, press `i` for insert mode, type some text, press Backspace

Expected

Backspace deletes the previous character.

Actual

Backspace inserts the ⌂ glyph (CP437 rendering of byte 0x7F / DEL).

Notes

• Vim launched directly from the same PowerShell session (& $Env:EDITOR) works correctly — Backspace deletes as expected. The bug only reproduces when Vim is spawned by Copilot CLI's external-editor feature.
• Apparent root cause: Copilot CLI puts the Windows console into VT input mode (ConPTY) for its own TUI. With ENABLE_VIRTUAL_TERMINAL_INPUT set, Backspace is delivered as 0x7F (^?, xterm style). Vim's Windows console build defaults to t_kb=^H (0x08), so it does not recognize 0x7F as Backspace and inserts it as a literal character (displayed as ⌂).
• The editor is spawned via plain stdio inheritance (no PTY bridge), so the parent's VT input mode leaks into the child.

Possible fixes? (Unverified)

• Allocate a PTY for the editor subprocess, or
• Temporarily clear ENABLE_VIRTUAL_TERMINAL_INPUT on the console before spawn and restore after, or
• Set TERM=win32 in the spawned editor's environment so Vim picks compatible termcap defaults.

Environment

• OS: Windows 11
• Shell: PowerShell
• Copilot CLI: 1.0.49-1
• Vim: 9.2 (console build, vim.exe)