Windows: Console window flash on subprocess spawn — root cause analysis and solutions (CREATE_NO_WINDOW, S4U, conhost --headless) #38715

Summary

On Windows, spawning console-subsystem processes (powershell.exe, pwsh.exe, cmd.exe, git.exe, etc.) causes a brief console window flash. This affects every bash tool invocation in opencode — each spawn of powershell.exe (and every child it launches) flashes a window. The Board has raised this repeatedly; root cause analysis below.

Root Cause

The fundamental mechanism: when Windows creates a console-subsystem process, conhost.exe allocates and draws the console window during process initialization in kernelbase.dll — before the application entry point runs and before any -WindowStyle Hidden flag or ShowWindow(SW_HIDE) call can execute. The window is on screen for a frame or two before being hidden.

Three specific failure modes:

1. Node.js child_process.spawn (Desktop app): Missing windowsHide: true + stdio: "pipe". Without these, Node passes CREATE_NEW_CONSOLE (or omits CREATE_NO_WINDOW), forcing a visible console allocation. Fix: spawn(cmd, args, { windowsHide: true, stdio: "pipe" }).

2. Go os/exec.Cmd (CLI/TUI, any Go-based tool spawn): Missing SysProcAttr.HideWindow = true and/or CREATE_NO_WINDOW (0x08000000) in ProcAttr.SysProcAttr.CreationFlags. Without these, Go's exec.Command inherits or allocates a visible console.

3. Scheduled Task launched as Interactive logon: powershell.exe -WindowStyle Hidden under Interactive logon still flashes. The "Hidden" flag only hides the window AFTER conhost has drawn it. Fix: set the task Principal to S4U ("Run whether user is logged on or not", LogonType S4U). Session-0 tasks never draw a window. Alternative: use conhost.exe --headless <cmd> as the task action for persistent runtime hosts.

Why -WindowStyle Hidden Is Not Sufficient

-WindowStyle Hidden is a runtime flag that PowerShell processes AFTER conhost.exe has already allocated the console window. The window is drawn, then hidden — but the flash has already occurred. This is a Windows architectural limitation, not a PowerShell bug (see microsoft/terminal#249 for the full discussion).

What "works" but has side effects

• VBScript shim (wscript.exe + CreateObject("WScript.Shell").Run(cmd, 0, False)): Does hide the console, but triggers a VBScriptDeprecationAlert (Event ID 4096) on every execution on modern Windows — a desktop dialog the user did not ask for. Deprecated approach.

Affected surfaces in opencode

• bash tool (every PowerShell invocation)
• git operations (diff, status, commit, etc.)
• MCP tools using .cmd scripts
• LSP server subprocesses
• File watchers and polling
• Any os/exec or child_process call to console-subsystem binaries

Recommended Fix Strategy

The key insight: this must be a default on Windows, not an opt-in per-call. Every shell-out to a console-subsystem binary that does not need a visible terminal should suppress the window at the OS level.

References

• Starting PowerShell/pwsh with '-WindowStyle hidden' should not flash console window. microsoft/terminal#249 — Windows console allocation cannot be deferred after process start
• Existing opencode issues: Windows: subprocess spawn (git, codegraph, etc.) flashes console window #31629 (open, same symptom), Desktop app: Console window flashes repeatedly even while idle (CREATE_NO_WINDOW missing) #22414 (stale-closed, same root cause)
• Node.js docs: windowsHide: true passes CREATE_NO_WINDOW to CreateProcessW
• Go docs: syscall.SysProcAttr.HideWindow / syscall.CREATE_NO_WINDOW on Windows