maxrave-dev Detecting a virtual machine on Windows
Some window features do not survive a VM's graphics driver. A transparent, undecorated window is the usual casualty: it never paints, the process stays alive, and the user sees nothing at all. The fix is to detect the VM and fall back to a normal decorated window — which makes the detection the load-bearing part.
The system fields that carry a hypervisor's brand live in the computer-system record of Windows' management layer. Read them, match against known vendor strings.
// adapted — extracted from the window-construction path, inside a `remember { }`
// Guard first: both probes below are Windows-only commands. Without this the
// app spawns a doomed subprocess on every macOS and Linux launch.
val osName = System.getProperty("os.name", "")
if (!osName.contains("Windows", ignoreCase = true)) return@remember false
val probes = listOf(
listOf(
"powershell", "-NoProfile", "-Command",
"(Get-CimInstance Win32_ComputerSystem | " +
"Select-Object Manufacturer,Model | " +
"Format-List | Out-String).Trim()",
),
listOf("wmic", "computersystem", "get", "manufacturer,model"), // older hosts only
)
val sysInfo = probes.asSequence()
.mapNotNull { cmd ->
runCatching {
val p = ProcessBuilder(cmd).redirectErrorStream(true).start()
val out = p.inputStream.bufferedReader().readText()
if (p.waitFor() == 0 && out.isNotBlank()) out else null
}.getOrNull()
}.firstOrNull().orEmpty()
val vmTokens = listOf("Parallels", "VirtualBox", "VMware", "QEMU", "KVM", "Xen", "Hyper-V")
val isVM = vmTokens.any { sysInfo.contains(it, ignoreCase = true) } ||
System.getProperty("app.window.no-transparent", "false").toBooleanStrictOrNull() == true
Then: Window(undecorated = !isVM, transparent = !isVM, …).
Traps
The classic command-line query tool is gone. It was deprecated in Windows 10 21H1 and is not installed by default on Windows 11 — a version-scoped fact, so keep it as the fallback probe rather than deleting it, since older hosts still have it and may not have a usable PowerShell path. On a modern host the old tool answers "not recognized", the probe reads back empty, and the code concludes "real hardware". This is exactly how a detection that shipped working degrades into a detection that has silently never run.
Probe BOTH the manufacturer and the model field. Which one carries the brand differs per hypervisor and per architecture — one product puts its company name in the manufacturer field and its product name in the model field; another puts a company name you would never guess in the manufacturer field and the recognisable brand only in the model. Reading one field is how a detection passes on your laptop's VM and fails on a user's.
Decide the fail direction on purpose, and know what it costs. Detection failure here resolves to "not a VM", which keeps the enhanced window. That is the right default for a hardened or unusual host that simply refuses to answer — it should not lose features for being quiet. But it also means a broken probe is indistinguishable from real hardware, and its symptom is the worst one available: an invisible window. If your fallback path is the cheap one instead, invert it. Either way, write down which way it fails and why.
Ship a manual override. No token list is exhaustive and no probe is guaranteed to answer. A system property (or an environment variable, or a settings toggle) that forces the fallback path turns "unusable, file a bug and wait for a release" into "add one flag". Note that the override in the excerpt only forces the safe direction — there is deliberately no way to force transparency on, because that is the state that renders nothing.
Run the probe once and remember it. It spawns a process. Doing that on every recomposition, or on every window resize, is a visible stall. Compute it once at window construction.
Redirect the error stream into the output you read, and require a zero exit and non-blank output. A probe that writes its complaint to standard error and exits zero otherwise looks like a successful empty answer.
Do not extend the token list by guessing. Vendor strings are not documented as an API and change between product generations.
Verifying it
The token list is data, not knowledge — regenerate it rather than trusting this file. On any host you care about, run the query itself and read what it prints:
Get-CimInstance Win32_ComputerSystem | Select-Object Manufacturer,Model | Format-List
Two checks that catch the failure this skill exists for:
- Prove the probe ran. Log the raw captured text, not just the boolean. An empty string with
isVM = falseand a real string withisVM = falseare entirely different situations and the boolean cannot tell them apart. - Prove the fallback works on a real guest, not on a resized window on your own machine. The render failure comes from the guest's graphics driver; nothing on the host reproduces it.