[Windows] Every shell command spawns a visible PowerShell console window (new console, not hidden) that stays open for the command's lifetime

Summary

On Windows, Copilot CLI briefly flashes an empty PowerShell / conhost console
window every time the agent executes a shell command. During normal agent
activity (many commands per task) this produces near-constant window flashing
that momentarily steals focus and is very distracting.

This appears to be the same class of bug as the already-fixed git / hook /
path-resolution flashes (PRs #10235 and #11037) and the open MCP-startup flash
(#3171), but on the general agent shell-command execution path, which is
still affected.

Environment

• Copilot CLI: 1.0.80 (latest at time of report)
• OS: Windows 11 24H2, build 26100.9106
  (registry ProductName cosmetically reads "Windows 10 Enterprise" on this build)
• Shell: PowerShell (default on Windows)
• Terminal: Windows Terminal

Steps to reproduce

1. Launch copilot in Windows Terminal on Windows 11.
2. Ask the agent to run essentially any shell command (e.g. a simple
   Get-ChildItem, or any node -e "..." invocation).
3. Watch the screen while the command executes.

Expected

Shell commands run without any visible console window appearing — as already
happens for git commands and for hook / path-resolution after #10235 / #11037.

Actual

A blank PowerShell / conhost window flashes on screen for each spawned
command, briefly stealing focus.

Real-world example that triggers it

This is an actual command a session spawned (a mermaid-diagram validation step).
Every command shaped like this flashes a window:

$node = "C:\ProgramData\global-npm\node_modules\node\bin\node.exe"
$script = @'
const fs = require("fs");
const files = process.argv.slice(2);
let idx = 0;
for (const f of files) {
  const txt = fs.readFileSync(f, "utf8").split(/\r?\n/);
  let inBlock = false, buf = [], start = 0;
  for (let i = 0; i < txt.length; i++) {
    const line = txt[i];
    if (!inBlock && /^\s*```mermaid\s*$/.test(line)) { inBlock = true; buf = []; start = i + 1; continue; }
    if (inBlock && /^\s*```\s*$/.test(line)) {
      inBlock = false; idx++;
      const base = f.replace(/^.*[\\/]/, "");
      const out = `${process.env.TEMP}\\mmvalidate\\block_${idx}_${base}_L${start+1}.mmd`;
      fs.writeFileSync(out, buf.join("\n"));
      continue;
    }
    if (inBlock) buf.push(line);
  }
}
'@
Set-Content -Path "$env:TEMP\mmvalidate\extract.js" -Value $script -Encoding utf8
& $node "$env:TEMP\mmvalidate\extract.js" "$env:TEMP\brga.md" "$env:TEMP\rad.md" "$env:TEMP\rrd.md"

Note: the window that flashes is the outer powershell.exe host the CLI
spawns to run the command — not node.exe. node is a console app and
inherits the parent console, so it does not create a window on its own. The
content of the command is irrelevant; any shell command triggers the flash.

Likely root cause

The CLI runs shell commands on Windows via node-pty / ConPTY (conpty.node).
The pseudo-console host (conhost.exe) for the spawned command is not being
suppressed on this path, so its window briefly appears. The earlier fixes added
window suppression (windowsHide / CREATE_NO_WINDOW) only to specific paths:

• #10235 — "Git commands no longer flash a console window on Windows"
• #11037 — "Prevent brief console windows from flashing on Windows when the
  agent runs hook commands or resolves command paths"

The main agent command-execution path appears not to have the equivalent
suppression.

Related

• #3171 — [Windows] CMD windows flash on screen when MCP servers start
  (same class of bug, different trigger)

Suggested fix

Apply the same window-suppression used in #10235 / #11037 to the agent's
shell-command execution path — i.e. ensure the ConPTY / child-process spawn
passes windowsHide: true (or creates the console host without a visible
window / uses a headless OpenConsole host consistently) for every command the
agent runs, not just git, hooks, and path resolution.

Suivez le chemin général d’exécution des commandes shell de l’agent et examinez comment il lance les commandes via node-pty/ConPTY. Comparez ses options de processus aux modifications de suppression des fenêtres des PRs #10235 et #11037, puis reproduisez le problème sous Windows ; le travail est terminé lorsque l’exécution répétée de commandes shell ne fait plus apparaître brièvement de fenêtre PowerShell ou conhost et ne vole plus le focus.