Stop the process-tree timeout test flashing a console window

[0101]

Problem

A terminal window periodically flashed on screen for about a second on my dev machine and vanished before it could be read. It turned out to be this test suite.

ProcessRunnerTests.fs — "timeout returns a typed error and terminates the process tree" — spawns ping -n 30 as a grandchild to prove ProcessRunner's deadline kills a whole process tree, not just the direct child:

$child = Start-Process ping -ArgumentList '127.0.0.1','-n','30' -PassThru; ...

Start-Process defaults to UseShellExecute = $true, so Windows allocates a new console for ping.exe. On Windows 11, where Windows Terminal is the default console host, that appears as a Windows Terminal window. Two seconds later the timeout kills the tree and the window disappears.

ProcessRunner already starts the outer powershell.exe with CreateNoWindow = true (ProcessRunner.fs), but that does not propagate to a grandchild launched through Start-Process — which is exactly the gap.

The test carries Category("Unit") / Category("Fast"), so unlike the other window-spawning fixtures (SessionManagerSpawnTests, ActionLaunchSpawnTests), it is not gated behind [<Explicit>] and runs in every normal test run. With several worktrees and background agents running the suite, the flashes appeared at unpredictable intervals.

I tracked this down with a process/window watcher, which caught the exact ancestry:

PROC powershell.exe(72720) cmd=["powershell" -NoProfile -Command
     "$child = Start-Process ping -ArgumentList '127.0.0.1','-n','30' -PassThru; ..."]
  PARENTCHAIN=
     testhost.exe    [src\Tests\bin\Debug\net10.0\testhost.exe]
  <= dotnet.exe      [vstest.console.dll --testCaseFilter:Category=Unit]
  <= dotnet.exe      [dotnet test src/Tests/Tests.fsproj --filter Category=Unit]

PROC conhost.exe(74004) PARENTCHAIN=powershell.exe(72720)

Fix

Add -NoNewWindow to the Start-Process call. ping then inherits the parent PowerShell's console — and since that parent is started with CreateNoWindow = true, there is no console to inherit and nothing is displayed.

-NoNewWindow is preferred over -WindowStyle Hidden: the latter is a display hint that only applies with UseShellExecute and is ignored by some hosts, whereas -NoNewWindow suppresses console allocation outright.

Everything the test asserts is unchanged: -PassThru still returns the child, the pid file is still written, and the parent/child process-tree relationship under test is untouched. The non-Windows sh/sleep 30 branch needed no change.

Tests

dotnet test src/Tests/Tests.fsproj --filter "FullyQualifiedName~ProcessRunner"

Passed! - Failed: 0, Passed: 15, Skipped: 0, Total: 15, Duration: 22 s

I also A/B tested the window suppression directly, reproducing how ProcessRunner starts the outer shell (UseShellExecute = false, CreateNoWindow = true) and counting visible console-class top-level windows across the run:

Variant	New visible windows	ping pid captured
before (no -NoNewWindow)	1	68132
after (-NoNewWindow)	0	45560

The window is gone, and the child pid is still captured — confirming the fix is cosmetic-only and the test's guarantees hold.

[Copilot]

Pull request overview

Prevents the Windows process-tree timeout test from flashing a console window.

Changes:

• Adds -NoNewWindow when spawning ping.
• Documents why console inheritance is required.

Show a summary per file

File	Description
src/Tests/ProcessRunnerTests.fs	Suppresses the temporary ping console window.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Medium