diff --git a/crates/ui-egui/tests/it/bookmark_position.rs b/crates/ui-egui/tests/it/bookmark_position.rs
index 03fd8b50a..09fd1d4be 100644
--- a/crates/ui-egui/tests/it/bookmark_position.rs
+++ b/crates/ui-egui/tests/it/bookmark_position.rs
@@ -55,21 +55,10 @@ fn harness(rotate: u16) -> Harness<'static, PdfCraftApp> {
         app
     });
     h.run_steps(4);
-    settle(&mut h);
+    crate::support::settle(&mut h);
     h
 }
 
-/// Run frames until rendering has finished and the layout holds still.
-fn settle(h: &mut Harness<'static, PdfCraftApp>) {
-    for _ in 0..200 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
-}
-
 /// Make a bookmark with ⌘B, as the Bookmarks button does, and say what it goes to.
 fn new_bookmark(h: &mut Harness<'static, PdfCraftApp>) -> (Option<usize>, DestView) {
     h.key_press_modifiers(Modifiers::COMMAND, Key::B);
@@ -103,7 +92,7 @@ fn a_new_bookmark_goes_to_the_top_of_the_window_not_the_top_of_the_page() {
     h.state_mut().apply_edit(Edit::AddBookmark { parent: vec![], index: 0, title: "Mid".into(), page: 1, left: None, top: Some(250.0) });
     h.run_steps(3);
     h.get_by_label("Mid").click();
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let (page, view) = new_bookmark(&mut h);
     assert_eq!(page, Some(1), "the page at the top of the window");
     let (left, top) = xyz(view);
@@ -151,7 +140,7 @@ fn a_bookmark_made_on_a_turned_page_pins_the_axis_the_window_scrolls_along() {
 fn rotate_view(h: &mut Harness<'static, PdfCraftApp>, degrees: u16) {
     h.state_mut().views[0].rotation = degrees;
     h.run_steps(3);
-    settle(h);
+    crate::support::settle(h);
 }
 
 #[test]
@@ -216,12 +205,12 @@ fn a_bookmark_in_a_spread_of_unequal_pages_goes_to_the_page_crossing_the_top_of_
     });
     h.state_mut().views[0].layout = PageLayout::TwoUp;
     h.run_steps(4);
-    settle(&mut h);
+    crate::support::settle(&mut h);
     // 20 pt below page 2's top is at the window's top, where page 1 has not started.
     h.state_mut().apply_edit(Edit::AddBookmark { parent: vec![], index: 0, title: "Right".into(), page: 1, left: None, top: Some(380.0) });
     h.run_steps(3);
     h.get_by_label("Right").click();
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let (page, view) = new_bookmark(&mut h);
     assert_eq!(page, Some(1), "page 2 crosses the top of the window; page 1 starts below it");
     close(xyz(view).1, 380.0, "top");
diff --git a/crates/ui-egui/tests/it/comments.rs b/crates/ui-egui/tests/it/comments.rs
index 6194c7516..68b41ac98 100644
--- a/crates/ui-egui/tests/it/comments.rs
+++ b/crates/ui-egui/tests/it/comments.rs
@@ -40,20 +40,10 @@ fn harness_with_bytes(bytes: Vec<u8>, setup: impl FnOnce(&mut PdfCraftApp) + 'st
         app
     });
     h.run_steps(4);
-    settle(&mut h);
+    crate::support::settle(&mut h);
     h
 }
 
-fn settle(h: &mut Harness<'static, PdfCraftApp>) {
-    for _ in 0..200 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
-}
-
 /// A point on page 1 in PDF user space → screen (no rotation; MediaBox 300×200).
 fn at(h: &Harness<'static, PdfCraftApp>, x: f32, y: f32) -> Pos2 {
     let r = h.state().views[0].page_screen_rect(0).expect("page 1 on screen");
@@ -146,7 +136,7 @@ fn sidebar_comment_centres_the_annotation_at_high_zoom() {
             && let Ok(dir) = std::env::var("PDFCRAFT_SHOTS")
         {
             let _gpu = crate::gpu();
-            settle(&mut h);
+            crate::support::settle(&mut h);
             h.render().unwrap().save(format!("{dir}/comment-centred.png")).unwrap();
         }
     }
@@ -754,7 +744,7 @@ fn a_dragged_text_box_follows_a_rotated_view() {
         }
         h.state_mut().set_option("zoom", "150").unwrap();
         h.run_steps(3);
-        settle(&mut h);
+        crate::support::settle(&mut h);
         let screen = |h: &Harness<'static, PdfCraftApp>, x: f32, y: f32| {
             let r = h.state().views[0].page_screen_rect(0).expect("page 1 on screen");
             let (u, v) = (x / 300.0, (200.0 - y) / 200.0);
@@ -1726,7 +1716,7 @@ fn attachment_harness(setup: impl FnOnce(&mut PdfCraftApp) + 'static) -> Harness
         app
     });
     h.run_steps(4);
-    settle(&mut h);
+    crate::support::settle(&mut h);
     h
 }
 
@@ -1915,7 +1905,7 @@ fn hovering_a_comment_shows_its_author_and_text() {
         app
     });
     h.run_steps(4);
-    settle(&mut h);
+    crate::support::settle(&mut h);
     assert!(h.query_by_label_contains("Check this figure").is_none(), "no popup before hovering");
     let p = at(&h, 100.0, 85.0);
     h.hover_at(p);
@@ -1945,7 +1935,7 @@ fn preference_hides_author_only_highlight_hover_previews() {
         app
     });
     h.run_steps(4);
-    settle(&mut h);
+    crate::support::settle(&mut h);
 
     let p = at(&h, 100.0, 85.0);
     // By default, author-only hover is shown.
@@ -1992,7 +1982,7 @@ fn markup_hover_harness(pdf: Vec<u8>, view_rotation: u16) -> Harness<'static, Pd
         app
     });
     h.run_steps(4);
-    settle(&mut h);
+    crate::support::settle(&mut h);
     h
 }
 
diff --git a/crates/ui-egui/tests/it/content.rs b/crates/ui-egui/tests/it/content.rs
index 515e3e04f..dc13bdb61 100644
--- a/crates/ui-egui/tests/it/content.rs
+++ b/crates/ui-egui/tests/it/content.rs
@@ -28,13 +28,7 @@ fn harness_bytes_with_step_dt(bytes: Vec<u8>, step_dt: f32) -> Harness<'static,
         app.set_option("zoom", "150").unwrap();
         app
     });
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     h
 }
 
@@ -161,13 +155,7 @@ fn text_tool_deleting_a_draft_hides_the_old_added_glyphs_and_cancel_restores_the
     click(&mut h, point);
     h.key_press_modifiers(egui::Modifiers::COMMAND, egui::Key::A);
     h.key_press(egui::Key::Backspace);
-    for _ in 0..100 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     assert!(h.state().views[0].content.draft.as_ref().unwrap().text.is_empty());
     let shot = h.render().unwrap();
     for x in 105..140 {
@@ -795,18 +783,6 @@ fn red(pixels: &image::RgbaImage, r: egui::Rect) -> usize {
         .count()
 }
 
-/// Steps until no render is pending.
-fn settle(h: &mut Harness<'static, PdfCraftApp>) {
-    for _ in 0..200 {
-        h.run_steps(1);
-        if !h.state().render_pending() {
-            return;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
-    panic!("the page did not finish rendering");
-}
-
 /// An added picture resized by a corner is drawn at its new size while dragged and after the
 /// release until the page is rendered again. It used to stay at its old size under the outline for
 /// the whole drag, and to show there again after the release.
@@ -826,7 +802,7 @@ fn a_resized_added_picture_shows_only_at_its_new_size() {
     h.state_mut().add_image("red.png".into(), png);
     h.run_steps(3);
     assert_eq!(h.state().views[0].content.selected, Some((0, 0)), "the picture is selected");
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let rect = added(&h)[0].content.rect();
     let r = item_on_screen(&h, rect);
     // The lower right corner to the middle: half the size, the upper left corner kept.
@@ -841,7 +817,7 @@ fn a_resized_added_picture_shows_only_at_its_new_size() {
         h.hover_at(grip + (to - grip) * (k as f32 / 4.0));
         h.run_steps(1);
     }
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let pixels = h.render().unwrap();
     assert!(red(&pixels, kept) > 100, "while dragged, the picture is drawn at its new size");
     assert_eq!(red(&pixels, gone), 0, "while dragged, the picture shows at its old size");
@@ -856,7 +832,7 @@ fn a_resized_added_picture_shows_only_at_its_new_size() {
     }
     assert_ne!(added(&h)[0].content.rect(), rect, "the resize is applied");
     h.state_mut().views[0].hold_rasters = false;
-    settle(&mut h);
+    crate::support::settle(&mut h);
     h.run_steps(2);
     let pixels = h.render().unwrap();
     assert!(red(&pixels, kept) > 100 && red(&pixels, gone) == 0, "the page raster has the picture at its new size");
diff --git a/crates/ui-egui/tests/it/control.rs b/crates/ui-egui/tests/it/control.rs
index 54bfe2fa3..ec29ecc97 100644
--- a/crates/ui-egui/tests/it/control.rs
+++ b/crates/ui-egui/tests/it/control.rs
@@ -73,13 +73,15 @@ fn harness_pages_at(pages: usize, width: f32, scale: f32) -> (Harness<'static, P
 /// Send a request and run frames until it is answered.
 fn call(h: &mut Harness<'static, PdfCraftApp>, c: &ControlClient, method: &str, params: Value) -> Reply {
     let rx = c.send(method, params);
-    for _ in 0..30 {
-        h.step();
-        if let Ok(r) = rx.try_recv() {
-            return r;
-        }
+    let mut reply = None;
+    let answered = crate::support::step_until(h, |_| {
+        reply = rx.try_recv().ok();
+        reply.is_some()
+    });
+    match reply {
+        Some(r) if answered => r,
+        _ => panic!("{method}: no reply in 30 s"),
     }
-    panic!("{method}: no reply after 30 frames");
 }
 
 fn ok(h: &mut Harness<'static, PdfCraftApp>, c: &ControlClient, method: &str, params: Value) -> Value {
@@ -1674,15 +1676,12 @@ fn control_open_replies_pending_while_the_worker_opens_and_focuses_once_it_has()
     // The worker completes the open on later frames; the tab shows, and only then does a
     // repeated open focus it instead of reporting another pending open.
     let mut documents = 0;
-    for _ in 0..30 {
-        h.run_steps(1);
-        let state = ok(&mut h, &c, "ui.state", json!({}));
+    let opened = crate::support::step_until(&mut h, |h| {
+        let state = ok(h, &c, "ui.state", json!({}));
         documents = state["documents"].as_array().map(|d| d.len()).unwrap_or_default();
-        if documents == 1 {
-            break;
-        }
-    }
-    assert_eq!(documents, 1, "the pending open completed into a tab");
+        documents == 1
+    });
+    assert!(opened, "the pending open completed into a tab: {documents} tabs");
 
     let again = ok(&mut h, &c, "ui.open", json!({ "path": a }));
     assert_eq!(again["focused"], true, "{again}");
diff --git a/crates/ui-egui/tests/it/editing.rs b/crates/ui-egui/tests/it/editing.rs
index 73464d33e..54b99d619 100644
--- a/crates/ui-egui/tests/it/editing.rs
+++ b/crates/ui-egui/tests/it/editing.rs
@@ -2826,13 +2826,7 @@ fn deleting_a_draft_does_not_show_the_original_text_underneath() {
     h.drop_at(at);
     h.run_steps(3);
     h.state_mut().views[0].line_editor.as_mut().unwrap().text.clear();
-    for _ in 0..100 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     let shot = h.render().unwrap();
     if let Ok(path) = std::env::var("PDFCRAFT_TEXT_PREVIEW_SHOT") {
         shot.save(path).unwrap();
@@ -2931,13 +2925,7 @@ fn opening_a_paragraph_leaves_the_page_as_it_is() {
     });
     h.run_steps(4);
     assert!(h.state_mut().execute("edit.edit_text"));
-    for _ in 0..100 {
-        h.run_steps(1);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     let page = h.state().views[0].page_screen_rect(0).expect("on screen");
     let k = page.width() / 300.0;
     let block = h.state().session.get(h.state().views[0].id).unwrap().text_blocks(0)[0].rect;
@@ -3008,13 +2996,7 @@ fn a_quick_click_into_another_paragraph_keeps_both_in_place() {
     };
     let (top, bottom) = (screen(&blocks[0]), screen(&blocks[1]));
     click_at(&mut h, top.center());
-    for _ in 0..100 {
-        h.run_steps(1);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     assert_eq!(h.state().views[0].line_editor.as_ref().map(|e| e.text.as_str()), Some("Top"));
     let ink = |pixels: &image::RgbaImage, r: egui::Rect| {
         (r.top() as u32..r.bottom() as u32)
@@ -4013,14 +3995,13 @@ fn save_paragraph_drag(h: &mut Harness<'static, PdfCraftApp>, name: &str, origin
 
 fn paragraph_drag_shot(h: &mut Harness<'static, PdfCraftApp>, name: &str) {
     if let Ok(directory) = std::env::var("PDFCRAFT_PARAGRAPH_DRAG_SHOTS") {
-        for _ in 0..60 {
-            h.run_steps(2);
-            if !h.state().render_pending() {
-                break;
-            }
-            std::thread::sleep(std::time::Duration::from_millis(5));
-        }
-        assert!(!h.state().render_pending(), "the screenshot must show the current document generation");
+        assert!(
+            crate::support::step_until(h, |h| {
+                h.run_steps(1);
+                !h.state().render_pending()
+            }),
+            "the screenshot must show the current document generation"
+        );
         let directory = std::path::Path::new(&directory);
         std::fs::create_dir_all(directory).unwrap();
         h.render().unwrap().save(directory.join(name)).unwrap();
@@ -5170,14 +5151,13 @@ fn image_pixels_follow_the_frame_while_a_corner_is_dragged() {
     h.run_steps(4);
     // The dragged pixels are the page raster's: wait for it, as a slow runner may not have it yet
     // (the drag then drew no picture at all).
-    for _ in 0..400 {
-        h.run_steps(1);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
-    assert!(!h.state().render_pending(), "the page raster arrives");
+    assert!(
+        crate::support::step_until(&mut h, |h| {
+            h.run_steps(1);
+            !h.state().render_pending()
+        }),
+        "the page raster arrives"
+    );
     let page = h.state().views[0].page_screen_rect(0).unwrap();
     let at = |x: f32, y: f32| egui::pos2(page.left() + x / 200.0 * page.width(), page.top() + (300.0 - y) / 300.0 * page.height());
     let p = at(95.0, 195.0);
@@ -5876,13 +5856,7 @@ fn a_paragraph_running_off_its_page_edits_without_covering_the_next_page() {
     h.drag_at(p);
     h.run_steps(1);
     h.drop_at(p);
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     assert!(
         h.state().views[0].line_editor.as_ref().is_some_and(|e| e.page == 0 && e.text.contains("line 50")),
         "the paragraph's off-page lines are in it"
@@ -5971,15 +5945,13 @@ fn mode_tabs_keep_the_hand_over_page_text_scrolled_under_them() {
     h.run_steps(4);
     let top = h.state().views[0].page_screen_rect(0).expect("on screen").top();
     let mut cursor = egui::CursorIcon::Default;
-    for _ in 0..50 {
+    let text_cursor = crate::support::step_until(&mut h, |h| {
         h.hover_at(text.center() + egui::vec2(0.0, top));
         h.run_steps(2);
         cursor = h.output().platform_output.cursor_icon;
-        if cursor == egui::CursorIcon::Text {
-            break;
-        }
-    }
-    assert_eq!(cursor, egui::CursorIcon::Text);
+        cursor == egui::CursorIcon::Text
+    });
+    assert!(text_cursor, "the I-beam over page text, not {cursor:?}");
     assert_eq!(mode_tab_cursors(&mut h, text), HAND_ON_EVERY_TAB);
 }
 
diff --git a/crates/ui-egui/tests/it/forms.rs b/crates/ui-egui/tests/it/forms.rs
index 230addd65..676a16db4 100644
--- a/crates/ui-egui/tests/it/forms.rs
+++ b/crates/ui-egui/tests/it/forms.rs
@@ -14,13 +14,7 @@ fn harness() -> Harness<'static, PdfCraftApp> {
         app.set_option("zoom", "150").unwrap();
         app
     });
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     h
 }
 
@@ -500,8 +494,13 @@ fn wait_for_file(h: &mut Harness<'static, PdfCraftApp>, path: &std::path::Path)
     h.run_steps(1);
 }
 
+/// A fresh directory of its own for each call. The tests share one process and run in parallel,
+/// so a name alone is not enough: two tests that both used "same-frame" deleted each other's
+/// directory mid-save, and the save then never wrote its file (#4186).
 fn scratch(name: &str) -> std::path::PathBuf {
-    let dir = std::env::temp_dir().join(format!("pdfcraft-forms-{name}-{}", std::process::id()));
+    static NEXT: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
+    let n = NEXT.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    let dir = std::env::temp_dir().join(format!("pdfcraft-forms-{name}-{}-{n}", std::process::id()));
     let _ = std::fs::remove_dir_all(&dir);
     std::fs::create_dir_all(&dir).unwrap();
     dir
@@ -1029,13 +1028,7 @@ fn calculated_field_and_script_side_effect_on_another_page_re_render() {
         event: "calculate".into(),
         script: Some("event.value = 'from_' + getField('name').value;".into()),
     }));
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     click_field(&mut h, "name", 0);
     h.event(egui::Event::Text("hello".into()));
     h.run_steps(2);
@@ -1043,13 +1036,7 @@ fn calculated_field_and_script_side_effect_on_another_page_re_render() {
     h.run_steps(2);
     assert_eq!(value(&h, "page2_field"), ["from_hello"]);
     // Wait for all re-renders to complete.
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     assert_eq!(value(&h, "name"), ["hello"]);
     assert_eq!(value(&h, "page2_field"), ["from_hello"]);
 }
@@ -1065,13 +1052,7 @@ fn overlay_clears_after_fresh_tiles_zoom_then_edit_and_rejected_edit() {
     h.key_press(egui::Key::Tab);
     h.run_steps(2);
     assert_eq!(value(&h, "name"), ["GoodVal"]);
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     assert!(!h.state().views[0].forms.unbaked.contains(&("name".to_string(), 0)));
 
     // 2. Rejected edit: typing rejected by validate script leaves unbaked clean.
@@ -1091,13 +1072,7 @@ fn overlay_clears_after_fresh_tiles_zoom_then_edit_and_rejected_edit() {
 
     // 3. Zoom then edit: zoom to another scale, let tiles settle, then edit.
     h.state_mut().views[0].zoom_step(true);
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     click_field(&mut h, "name", 0);
     h.key_press_modifiers(egui::Modifiers::COMMAND, egui::Key::A);
     h.event(egui::Event::Text("ZoomedVal".into()));
@@ -1105,13 +1080,7 @@ fn overlay_clears_after_fresh_tiles_zoom_then_edit_and_rejected_edit() {
     h.key_press(egui::Key::Tab);
     h.run_steps(2);
     assert_eq!(value(&h, "name"), ["ZoomedVal"]);
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     assert!(!h.state().views[0].forms.unbaked.contains(&("name".to_string(), 0)));
 }
 
diff --git a/crates/ui-egui/tests/it/object_selection.rs b/crates/ui-egui/tests/it/object_selection.rs
index 6e428ad74..fc2a8e4fe 100644
--- a/crates/ui-egui/tests/it/object_selection.rs
+++ b/crates/ui-egui/tests/it/object_selection.rs
@@ -73,13 +73,7 @@ fn harness_document(clipping: bool, background: bool) -> Harness<'static, PdfCra
         assert!(app.execute("edit.edit_text"));
         app
     });
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     h
 }
 
diff --git a/crates/ui-egui/tests/it/ocr.rs b/crates/ui-egui/tests/it/ocr.rs
index 20fb73344..7f2e3bdc5 100644
--- a/crates/ui-egui/tests/it/ocr.rs
+++ b/crates/ui-egui/tests/it/ocr.rs
@@ -133,13 +133,7 @@ fn an_ocr_word_opens_for_editing_and_stays_open() {
         assert!(app.execute("edit.edit_text"));
         app
     });
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     let (r, k) = {
         let app = h.state();
         let d = app.session.get(app.views[0].id).unwrap();
diff --git a/crates/ui-egui/tests/it/pickers.rs b/crates/ui-egui/tests/it/pickers.rs
index e4ce56d7c..bbab087d9 100644
--- a/crates/ui-egui/tests/it/pickers.rs
+++ b/crates/ui-egui/tests/it/pickers.rs
@@ -115,9 +115,12 @@ fn a_pick_for_a_document_that_is_no_longer_active_is_dropped() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
-/// A scratch folder for one test.
+/// A scratch folder for one test, of its own for each call: the tests share one process and run
+/// in parallel, so two that picked the same name would delete each other's folder (#4186).
 fn scratch(name: &str) -> std::path::PathBuf {
-    let dir = std::env::temp_dir().join(format!("pdfcraft-pickers-{name}-{}", std::process::id()));
+    static NEXT: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
+    let n = NEXT.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    let dir = std::env::temp_dir().join(format!("pdfcraft-pickers-{name}-{}-{n}", std::process::id()));
     let _ = std::fs::remove_dir_all(&dir);
     std::fs::create_dir_all(&dir).unwrap();
     dir
diff --git a/crates/ui-egui/tests/it/prepare.rs b/crates/ui-egui/tests/it/prepare.rs
index 106d2d539..bf0f6c68d 100644
--- a/crates/ui-egui/tests/it/prepare.rs
+++ b/crates/ui-egui/tests/it/prepare.rs
@@ -17,13 +17,7 @@ fn harness() -> Harness<'static, PdfCraftApp> {
         app.set_option("zoom", "150").unwrap();
         app
     });
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     h
 }
 
diff --git a/crates/ui-egui/tests/it/redact.rs b/crates/ui-egui/tests/it/redact.rs
index c2d150e11..3e11861cb 100644
--- a/crates/ui-egui/tests/it/redact.rs
+++ b/crates/ui-egui/tests/it/redact.rs
@@ -14,13 +14,7 @@ fn harness() -> Harness<'static, PdfCraftApp> {
         app.set_option("zoom", "150").unwrap();
         app
     });
-    for _ in 0..60 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     h
 }
 
diff --git a/crates/ui-egui/tests/it/support/mod.rs b/crates/ui-egui/tests/it/support/mod.rs
index c4c0ee060..2e1e11ffb 100644
--- a/crates/ui-egui/tests/it/support/mod.rs
+++ b/crates/ui-egui/tests/it/support/mod.rs
@@ -2,21 +2,43 @@
 use egui_kittest::Harness;
 use pdfcraft_ui_egui::PdfCraftApp;
 
-pub fn wait_until(h: &mut Harness<'static, PdfCraftApp>, what: &str, ready: impl Fn(&PdfCraftApp) -> bool) {
+/// Step frames until `ready` holds or 30 seconds pass, and say whether it held. For background work
+/// (a worker-thread open, page text arriving) that a test then asserts on itself: a frame count is
+/// not a time, and a busy runner may not have finished the work within one. Sleeps briefly between
+/// frames so the worker threads get the CPU.
+#[must_use]
+pub fn step_until(h: &mut Harness<'static, PdfCraftApp>, mut ready: impl FnMut(&mut Harness<'static, PdfCraftApp>) -> bool) -> bool {
     let deadline = std::time::Instant::now() + std::time::Duration::from_secs(30);
     loop {
         h.step();
-        if ready(h.state()) {
-            return;
+        if ready(h) {
+            return true;
+        }
+        if std::time::Instant::now() >= deadline {
+            return false;
         }
-        assert!(
-            std::time::Instant::now() < deadline,
-            "timed out waiting for {what}: views={}, active={:?}, password={:?}, toast={:?}",
-            h.state().views.len(),
-            h.state().active,
-            h.state().password_prompt.as_ref().map(|p| (&p.name, &p.error)),
-            h.state().toast
-        );
         std::thread::sleep(std::time::Duration::from_millis(5));
     }
 }
+
+/// Step frames until nothing is waiting to be rendered, and fail the test if that takes over 30
+/// seconds: what a test checks next is on the rendered page.
+pub fn settle(h: &mut Harness<'static, PdfCraftApp>) {
+    let settled = step_until(h, |h| {
+        h.run_steps(1);
+        !h.state().render_pending()
+    });
+    assert!(settled, "pages rendered");
+}
+
+pub fn wait_until(h: &mut Harness<'static, PdfCraftApp>, what: &str, ready: impl Fn(&PdfCraftApp) -> bool) {
+    let held = step_until(h, |h| ready(h.state()));
+    assert!(
+        held,
+        "timed out waiting for {what}: views={}, active={:?}, password={:?}, toast={:?}",
+        h.state().views.len(),
+        h.state().active,
+        h.state().password_prompt.as_ref().map(|p| (&p.name, &p.error)),
+        h.state().toast
+    );
+}
diff --git a/crates/ui-egui/tests/it/ui.rs b/crates/ui-egui/tests/it/ui.rs
index 6472b7c66..275cdab76 100644
--- a/crates/ui-egui/tests/it/ui.rs
+++ b/crates/ui-egui/tests/it/ui.rs
@@ -451,7 +451,7 @@ fn overflowing_tabs_can_be_reached_by_scrolling() {
     assert!(h.state().active.is_some_and(|i| i < 19));
     if let Ok(dir) = std::env::var("PDFCRAFT_TAB_SHOTS") {
         let _gpu = crate::gpu();
-        settle(&mut h);
+        crate::support::settle(&mut h);
         h.render().unwrap().save(format!("{dir}/overflow-tabs.png")).unwrap();
     }
 }
@@ -565,7 +565,7 @@ fn narrow_tab_strips_accept_vertical_wheel_scrolling() {
         assert!(h.get_by_label_contains("Display theme:").rect().right() <= width);
         if let Ok(dir) = std::env::var("PDFCRAFT_TAB_SHOTS") {
             let _gpu = crate::gpu();
-            settle(&mut h);
+            crate::support::settle(&mut h);
             h.render().unwrap().save(format!("{dir}/tabs-{width}.png")).unwrap();
         }
     }
@@ -612,23 +612,13 @@ endstream endobj
 trailer << /Root 1 0 R >>
 %%EOF";
 
-fn settle(h: &mut Harness<'static, PdfCraftApp>) {
-    for _ in 0..200 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
-}
-
 #[test]
 fn find_counts_matches_across_pages() {
     let mut h = harness(|app| {
         app.open_bytes("text.pdf", None, TEXT_FIXTURE.to_vec()).expect("opens");
         app.set_option("find", "brown").unwrap();
     });
-    settle(&mut h);
+    crate::support::settle(&mut h);
     h.get_by_label_contains("of 2");
     let v = &h.state().views[0];
     let f = v.find.as_ref().expect("find open");
@@ -641,7 +631,7 @@ fn find_reports_no_matches() {
         app.open_bytes("text.pdf", None, TEXT_FIXTURE.to_vec()).expect("opens");
         app.set_option("find", "zebra").unwrap();
     });
-    settle(&mut h);
+    crate::support::settle(&mut h);
     h.get_by_label("No matches");
 }
 
@@ -651,7 +641,7 @@ fn drag_selects_text_and_copy_returns_it() {
         app.open_bytes("text.pdf", None, TEXT_FIXTURE.to_vec()).expect("opens");
         app.set_option("left", "closed").unwrap();
     });
-    settle(&mut h);
+    crate::support::settle(&mut h);
     // "The quick brown fox": glyph 4 = 'q', glyph 14 = 'n' of "brown".
     let (a, b) = {
         let v = &h.state().views[0];
@@ -738,7 +728,7 @@ fn zoom_keeps_the_point_under_the_cursor_still() {
         app.set_option("left", "closed").unwrap();
         app.set_option("panel", "none").unwrap();
     });
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let before = h.state().views[0].glyph_screen_pos(0, 4).expect("glyph on screen");
     let z = h.state().views[0].zoom;
     h.state_mut().views[0].zoom_at(z * 2.0, before);
@@ -754,7 +744,7 @@ fn trackpad_pinch_zooms_the_pdf_under_the_pointer() {
         app.set_option("left", "closed").unwrap();
         app.set_option("panel", "none").unwrap();
     });
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let before = h.state().views[0].glyph_screen_pos(0, 4).expect("glyph on screen");
     let zoom = h.state().views[0].zoom;
     h.hover_at(before);
@@ -781,7 +771,7 @@ fn selection_works_in_every_view_rotation() {
             // Keep the whole rotated page on screen.
             app.set_option("zoom", "60").unwrap();
         });
-        settle(&mut h);
+        crate::support::settle(&mut h);
         let (a, b) = {
             let v = &h.state().views[0];
             (v.glyph_screen_pos(0, 4).expect("glyph"), v.glyph_screen_pos(0, 14).expect("glyph"))
@@ -1321,7 +1311,7 @@ fn document_tabs_expose_selection_and_activate_from_keyboard() {
         }
         if let Ok(dir) = std::env::var("PDFCRAFT_TAB_SHOTS") {
             let _gpu = crate::gpu();
-            settle(&mut h);
+            crate::support::settle(&mut h);
             h.render().unwrap().save(format!("{dir}/focused-tab-{key:?}.png")).unwrap();
         }
     }
@@ -1547,7 +1537,7 @@ fn short_window_rail_buttons_scroll_clear_of_page_navigation() {
         app.open_bytes("short.pdf", None, FIXTURE.to_vec()).unwrap();
         app
     });
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let rail = |h: &Harness<'static, PdfCraftApp>, label: &str| {
         h.query_all_by_label(label).map(|n| n.rect()).find(|r| r.left() >= width - 48.0).unwrap_or_else(|| panic!("{label} in the rail"))
     };
diff --git a/crates/ui-egui/tests/it/view.rs b/crates/ui-egui/tests/it/view.rs
index dc1d5585d..3520f0a98 100644
--- a/crates/ui-egui/tests/it/view.rs
+++ b/crates/ui-egui/tests/it/view.rs
@@ -1266,13 +1266,7 @@ fn highlight_fields_tints_the_field_area() {
     h.state_mut().set_option("panel", "none").unwrap();
     // Highlighting is on by default; start from plain fields.
     h.state_mut().set_option("fields", "off").unwrap();
-    for _ in 0..100 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     let r = rect(&h, 0).expect("page 1");
     // The field spans x 50..250, y 300..330 (PDF space, y up) on a 300×400 page; sample inside it.
     let at = egui::pos2(r.min.x + r.width() * (60.0 / 300.0), r.min.y + r.height() * (1.0 - 305.0 / 400.0));
@@ -1409,13 +1403,7 @@ fn required_radio_buttons_get_a_round_red_border() {
         app.set_option("fields", "on").unwrap();
         app
     });
-    for _ in 0..100 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     let r = rect(&h, 0).expect("page 1");
     let ppp = h.ctx.pixels_per_point();
     let img = h.render().expect("renders");
@@ -1560,13 +1548,7 @@ fn required_fields_get_a_red_border_when_highlighting() {
     h.state_mut().set_option("fields", "on").unwrap();
     let props = pdfcraft_engine::FieldProps { required: Some(true), ..Default::default() };
     h.state_mut().apply_edit(pdfcraft_engine::Edit::SetFieldProps { name: "fullname".into(), props: Box::new(props) });
-    for _ in 0..100 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     let r = rect(&h, 0).expect("page 1");
     // Just inside the left edge of the field (x 50, y 300..330 on a 300×400 page).
     let ppp = h.ctx.pixels_per_point();
@@ -1622,13 +1604,7 @@ trailer << /Root 1 0 R >>
 /// Mean and largest difference between page 1 on screen and the page rendered directly at the
 /// view's exact device scale, over the lined area of `page_raster_maps_one_to_one_onto_screen_pixels`.
 fn compare_page_with_raster(h: &mut Harness<'static, PdfCraftApp>, pdf: &[u8], zoom: f32) -> (f64, u8) {
-    for _ in 0..100 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(h);
     let ppp = h.ctx.pixels_per_point();
     let r = rect(h, 0).expect("page 1");
     let img = h.render().expect("renders");
@@ -1693,16 +1669,6 @@ fn grid_harness(pdf: Vec<u8>, zoom: &str) -> Harness<'static, PdfCraftApp> {
     })
 }
 
-fn settle(h: &mut Harness<'static, PdfCraftApp>) {
-    for _ in 0..400 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
-}
-
 /// Detail in the middle of the window: the summed difference between neighbouring pixels.
 fn centre_detail(h: &mut Harness<'static, PdfCraftApp>) -> u64 {
     let img = h.render().expect("renders");
@@ -1720,7 +1686,7 @@ fn centre_detail(h: &mut Harness<'static, PdfCraftApp>) -> u64 {
 fn zooming_a_tiled_page_shows_its_earlier_tiles_until_new_ones_arrive() {
     let _gpu = gpu();
     let mut h = grid_harness(fine_grid_pdf(), "300");
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let sharp = centre_detail(&mut h);
     // The first frame at a new zoom has no tiles for it yet (they are requested at its end).
     h.state_mut().set_option("zoom", "310").unwrap();
@@ -1736,13 +1702,13 @@ fn zooming_a_tiled_page_shows_its_earlier_tiles_until_new_ones_arrive() {
 fn returning_to_a_visited_zoom_reuses_its_cached_tiles() {
     let _gpu = gpu();
     let mut h = grid_harness(fine_grid_pdf(), "300");
-    settle(&mut h);
+    crate::support::settle(&mut h);
     let before = h.state().views[0].tiles_debug(0);
     assert!(!before.is_empty(), "the 300 % page should be tiled, but it cached no tiles");
     // Zoom right out, where the 1200 pt page fits in one raster and is not tiled at all, so
     // none of its tiles is demanded...
     h.state_mut().set_option("zoom", "50").unwrap();
-    settle(&mut h);
+    crate::support::settle(&mut h);
     // ...then back to the first zoom, which was already rendered sharp.
     h.state_mut().set_option("zoom", "300").unwrap();
     h.step();
diff --git a/crates/ui-egui/tests/it/viewer.rs b/crates/ui-egui/tests/it/viewer.rs
index a3c86fc52..8052e6a31 100644
--- a/crates/ui-egui/tests/it/viewer.rs
+++ b/crates/ui-egui/tests/it/viewer.rs
@@ -40,13 +40,7 @@ fn harness() -> Harness<'static, PdfCraftApp> {
         app
     });
     // Background rendering and text extraction can take seconds on a loaded CI runner (FreeBSD VM).
-    for _ in 0..3000 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     h
 }
 
@@ -1732,13 +1726,7 @@ fn a_picture_inside_form_artwork_under_text_is_selected_and_copied() {
         app.open_bytes("scan.pdf", None, pdf).unwrap();
         app
     });
-    for _ in 0..40 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
+    crate::support::settle(&mut h);
     h.state_mut().system_clipboard = false;
     let i = h.state().active.unwrap();
     let r = h.state().views[i].page_screen_rect(0).expect("on screen");
diff --git a/crates/ui-egui/tests/it/xfa.rs b/crates/ui-egui/tests/it/xfa.rs
index cf0fc193b..6db98a52b 100644
--- a/crates/ui-egui/tests/it/xfa.rs
+++ b/crates/ui-egui/tests/it/xfa.rs
@@ -103,17 +103,6 @@ fn ordinary_forms_and_documents_have_no_xfa_notice() {
     assert_eq!(xfa(&h), Some(pdfcraft_render::Xfa::Dynamic));
 }
 
-/// Run frames until the pages are drawn (fields can be clicked then).
-fn wait_for_pages(h: &mut Harness<'static, PdfCraftApp>) {
-    for _ in 0..40 {
-        h.run_steps(2);
-        if !h.state().render_pending() {
-            break;
-        }
-        std::thread::sleep(std::time::Duration::from_millis(5));
-    }
-}
-
 /// Click the centre of a field's widget.
 fn click_field(h: &mut Harness<'static, PdfCraftApp>, name: &str) {
     let p = {
@@ -133,7 +122,7 @@ fn click_field(h: &mut Harness<'static, PdfCraftApp>, name: &str) {
 #[test]
 fn xfa_buttons_run_their_scripts_in_the_app() {
     let mut h = open(pdfcraft_xfa::fixtures::shell(&pdfcraft_xfa::fixtures::scripted_template()));
-    wait_for_pages(&mut h);
+    crate::support::settle(&mut h);
     let names = |h: &Harness<'static, PdfCraftApp>| {
         let s = h.state();
         s.session.get(s.views[0].id).unwrap().form.iter().map(|f| f.name.clone()).collect::<Vec<_>>()
@@ -189,7 +178,7 @@ fn a_form_whose_scripts_were_turned_off_at_open_says_so() {
 fn scripts_turned_off_by_a_click_bring_a_closed_notice_bar_back() {
     let tpl = pdfcraft_xfa::fixtures::scripted_template().replace(r#"xfa.host.messageBox("Hello " + qty.rawValue);"#, RUNAWAY);
     let mut h = open(pdfcraft_xfa::fixtures::shell(&tpl));
-    wait_for_pages(&mut h);
+    crate::support::settle(&mut h);
     // The form opens with its scripts on, and the user closes the notice bar.
     let id = h.state().views[0].id;
     assert!(!h.state().session.get(id).unwrap().xfa_scripts_off());