diff --git a/crates/ui-egui/src/render.rs b/crates/ui-egui/src/render.rs
--- a/crates/ui-egui/src/render.rs
+++ b/crates/ui-egui/src/render.rs
@@ -267,10 +267,28 @@ impl Slot {
     }
 }
 
+/// Render workers of an app built by this crate's own tests. Most of them build an app, several
+/// run at once, and a test needs a render or two at a time, not the machine's six (issue #3067:
+/// sixteen tests at once ran about 96 render threads on 16 cores). A test that needs more asks
+/// for them with [`Renderer::with_worker_threads`].
+#[cfg(test)]
+pub(crate) const TEST_WORKERS: usize = 2;
+
+/// Render workers an app starts with: the machine's job-pool size, at most six (none on wasm).
+#[cfg(not(test))]
+fn default_workers() -> usize {
+    if cfg!(target_arch = "wasm32") { 0 } else { JobPool::<Slot, Finished>::default_threads().min(6) }
+}
+
+/// Render workers an app starts with in this crate's tests: [`TEST_WORKERS`].
+#[cfg(test)]
+fn default_workers() -> usize {
+    TEST_WORKERS
+}
+
 impl Default for Renderer {
     fn default() -> Self {
-        let threads = if cfg!(target_arch = "wasm32") { 0 } else { JobPool::<Slot, Finished>::default_threads().min(6) };
-        Self::with_worker_threads(threads)
+        Self::with_worker_threads(default_workers())
     }
 }
 
diff --git a/crates/ui-egui/src/render/thumbnail_tests.rs b/crates/ui-egui/src/render/thumbnail_tests.rs
--- a/crates/ui-egui/src/render/thumbnail_tests.rs
+++ b/crates/ui-egui/src/render/thumbnail_tests.rs
@@ -446,6 +446,25 @@ fn a_panicking_render_is_a_failure_not_a_dead_worker() {
     assert_eq!(app.renderer.live_workers(), workers);
 }
 
+/// An app built by this crate's tests renders on two workers, not the machine's six: several
+/// tests run at once, and each of them used to start six (issue #3067).
+#[test]
+fn a_test_app_renders_on_two_workers() {
+    let (mut app, _, ctx) = fixture_with(Session::new(), (*photo()).clone());
+    app.renderer.offload = None; // the real job pool
+    let slot = Slot::Thumb(PhotoId(1));
+    let job = app.session.thumb_job(PhotoId(1), 256).unwrap();
+    app.renderer.request(slot, job, 30);
+    assert_eq!(app.renderer.live_workers(), TEST_WORKERS, "the pool starts all its workers with its first job");
+    let t0 = std::time::Instant::now();
+    while app.renderer.is_pending(slot) {
+        assert!(t0.elapsed() < std::time::Duration::from_secs(20), "the render never answered");
+        std::thread::sleep(std::time::Duration::from_millis(5));
+        app.renderer.poll(&ctx, &mut app.session);
+    }
+    assert!(app.renderer.textures.contains_key(&slot), "{:?}", app.renderer.failure(slot));
+}
+
 #[test]
 fn raw_quick_path_frames_build_no_jobs() {
     // an unedited raw: the grid shows its embedded preview (held here until released) and stops