diff --git a/apps/vectorcraft/src/gpu/tests.rs b/apps/vectorcraft/src/gpu/tests.rs
index 906e35fb6..d13a1c417 100644
--- a/apps/vectorcraft/src/gpu/tests.rs
+++ b/apps/vectorcraft/src/gpu/tests.rs
@@ -548,11 +548,14 @@ fn a_normal_exit_or_another_failure_does_not_restart() {
     assert_eq!(finish(Err("a bug".into()), &startup), Err("a bug".into()));
 }
 
-/// A log folder of its own for each test.
-fn start_dir(tag: &str) -> std::path::PathBuf {
+/// A log folder of its own for each test, and the guard that keeps other tests from starting a
+/// program while it runs: the test releases the start file's lock and takes it again, and a child
+/// starting meanwhile would keep it held (`crate::no_spawns`).
+fn start_dir(tag: &str) -> (std::path::PathBuf, std::sync::RwLockReadGuard<'static, ()>) {
+    let spawns = crate::no_spawns();
     let d = std::env::temp_dir().join(format!("vectorcraft-gpu-start-{tag}-{}", std::process::id()));
     let _ = std::fs::remove_dir_all(&d);
-    d
+    (d, spawns)
 }
 
 /// What a start in `dir` leaves out; it then ends without crashing.
@@ -578,7 +581,7 @@ fn issue_2243() -> Vec<Candidate> {
 /// it, without a status-bar note.
 #[test]
 fn a_start_that_never_showed_a_frame_leaves_its_adapter_out_of_the_next() {
-    let dir = start_dir("crash");
+    let (dir, _spawns) = start_dir("crash");
     let c = issue_2243();
     let first = Startup::default();
     assert_eq!(first.track(&dir, None, false), LeftOut::default());
@@ -609,7 +612,7 @@ fn a_start_that_never_showed_a_frame_leaves_its_adapter_out_of_the_next() {
 /// an error, a restart on another adapter), leaves nothing out of the next.
 #[test]
 fn a_start_that_showed_its_first_frame_or_ended_is_cleared() {
-    let dir = start_dir("shown");
+    let (dir, _spawns) = start_dir("shown");
     let c = issue_2243();
     for end in [Startup::presented as fn(&Startup), Startup::ended] {
         let first = Startup::default();
@@ -633,7 +636,7 @@ fn a_start_that_showed_its_first_frame_or_ended_is_cleared() {
 /// macOS, where wgpu has none.
 #[test]
 fn a_crash_while_listing_lists_opengl_alone() {
-    let dir = start_dir("listing");
+    let (dir, _spawns) = start_dir("listing");
     let first = Startup::default();
     first.track(&dir, None, false);
     drop(first);
@@ -660,7 +663,7 @@ fn a_crash_while_listing_lists_opengl_alone() {
 /// start shows its first frame on the adapter that crashed, the adapter counts again.
 #[test]
 fn wgpu_backend_or_adapter_name_ignores_the_crashes() {
-    let dir = start_dir("explicit");
+    let (dir, _spawns) = start_dir("explicit");
     let c = issue_2243();
     let first = Startup::default();
     first.track(&dir, None, false);
@@ -681,7 +684,7 @@ fn wgpu_backend_or_adapter_name_ignores_the_crashes() {
 /// are ignored: the start goes on with nothing left out.
 #[test]
 fn a_damaged_or_unreadable_start_file_is_ignored() {
-    let dir = start_dir("damaged");
+    let (dir, _spawns) = start_dir("damaged");
     std::fs::create_dir_all(&dir).unwrap();
     let damaged: [&[u8]; 9] = [
         b"",
diff --git a/apps/vectorcraft/src/main.rs b/apps/vectorcraft/src/main.rs
index c37aebe96..926f30b25 100644
--- a/apps/vectorcraft/src/main.rs
+++ b/apps/vectorcraft/src/main.rs
@@ -483,5 +483,28 @@ fn main() -> std::process::ExitCode {
     }
 }
 
+/// Tests only: while a test starts another program, the child briefly holds a copy of every file
+/// this process has open, until it runs that program. A file lock held through such a copy outlives
+/// its own handle, so a GPU start test that released the start file and opened it again found it
+/// still locked, as if another copy of the app were running, and failed. A test that starts a
+/// program holds [`spawning`]; a test that depends on the start file's lock holds [`no_spawns`].
+#[cfg(all(
+    test,
+    any(feature = "wgpu", any(target_os = "linux", target_os = "freebsd", target_os = "dragonfly", target_os = "netbsd", target_os = "openbsd"))
+))]
+static CHILD_SPAWN: std::sync::RwLock<()> = std::sync::RwLock::new(());
+
+/// Held while a test starts another program (see [`CHILD_SPAWN`]).
+#[cfg(all(test, any(target_os = "linux", target_os = "freebsd", target_os = "dragonfly", target_os = "netbsd", target_os = "openbsd")))]
+fn spawning() -> std::sync::RwLockWriteGuard<'static, ()> {
+    CHILD_SPAWN.write().unwrap_or_else(std::sync::PoisonError::into_inner)
+}
+
+/// Held by a test that releases a locked file and opens it again (see [`CHILD_SPAWN`]).
+#[cfg(all(test, feature = "wgpu"))]
+fn no_spawns() -> std::sync::RwLockReadGuard<'static, ()> {
+    CHILD_SPAWN.read().unwrap_or_else(std::sync::PoisonError::into_inner)
+}
+
 #[cfg(all(test, feature = "wgpu"))]
 mod tests;
diff --git a/apps/vectorcraft/src/portal_save.rs b/apps/vectorcraft/src/portal_save.rs
index 10bc2d8a8..b6dd3cc8e 100644
--- a/apps/vectorcraft/src/portal_save.rs
+++ b/apps/vectorcraft/src/portal_save.rs
@@ -450,11 +450,13 @@ mod tests {
         use std::io::BufRead as _;
 
         use zbus::zvariant::OwnedObjectPath;
-        let mut daemon = std::process::Command::new("dbus-daemon")
+        let spawning = crate::spawning();
+        let daemon = std::process::Command::new("dbus-daemon")
             .args(["--session", "--nofork", "--print-address=1"])
             .stdout(std::process::Stdio::piped())
-            .spawn()
-            .ok()?;
+            .spawn();
+        drop(spawning);
+        let mut daemon = daemon.ok()?;
         let mut address = String::new();
         std::io::BufReader::new(daemon.stdout.take()?).read_line(&mut address).ok()?;
         let address = address.trim().to_string();