diff --git a/apps/solidcraft-cli/src/main.rs b/apps/solidcraft-cli/src/main.rs
index 8d9db6ff..15aa2883 100644
--- a/apps/solidcraft-cli/src/main.rs
+++ b/apps/solidcraft-cli/src/main.rs
@@ -410,10 +410,13 @@ fn cmd_bench_open(args: &[String]) -> Result<(), String> {
     let read_ms = t.elapsed().as_secs_f64() * 1000.0;
     println!("{}", json!({"step": "read", "ms": read_ms, "bytes": bytes.len()}));
     let t = std::time::Instant::now();
+    // The file as read, held until the timeline has evaluated it (as the import command does).
+    let mut read = None;
     let doc = if step {
         let imported = solidcraft_engine::io::step_import_feature(&bytes, path).map_err(|e| e.to_string())?;
         let mut doc = solidcraft_engine::doc::Document::new(&imported.name);
         doc.add_feature(imported.kind, Some(&imported.name)).map_err(|e| e.to_string())?;
+        read = Some(imported.import);
         doc
     } else {
         solidcraft_engine::io::read_design(&bytes).map_err(|e| e.to_string())?
@@ -425,6 +428,7 @@ fn cmd_bench_open(args: &[String]) -> Result<(), String> {
     }
     let t = std::time::Instant::now();
     let s = Session::new(doc);
+    drop(read);
     println!("{}", json!({"step": "evaluate", "ms": t.elapsed().as_secs_f64() * 1000.0}));
     for r in &s.model.results {
         println!("{}", json!({"step": "feature", "id": r.id, "name": r.name, "ms": r.ms, "error": r.error}));
diff --git a/crates/doc/src/document.rs b/crates/doc/src/document.rs
index 41868e2d..10c63384 100644
--- a/crates/doc/src/document.rs
+++ b/crates/doc/src/document.rs
@@ -1032,8 +1032,9 @@ pub enum FeatureKind {
     Import {
         /// Source file name (display only).
         file: String,
-        /// The STEP (ISO 10303-21) text.
-        step: String,
+        /// The STEP (ISO 10303-21) text, shared by every copy of the design (undo states,
+        /// previews, scratch sessions): a large file is held once, not once per edit.
+        step: std::sync::Arc<str>,
         /// The file's product/assembly tree, kept for components.
         #[serde(default, skip_serializing_if = "Vec::is_empty")]
         components: Vec<solidcraft_kernel::StepNode>,
diff --git a/crates/doc/src/eval.rs b/crates/doc/src/eval.rs
index 9146d40a..a7e61b10 100644
--- a/crates/doc/src/eval.rs
+++ b/crates/doc/src/eval.rs
@@ -4407,9 +4407,12 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
         FeatureKind::Import { step, .. } => {
             let imp = kernel::step_import_shared(step)?;
             let digest = crate::config::payload_digest(step);
+            // Files from board and assembly tools give thousands of bodies the same name: name
+            // them from a set, not by searching the bodies for every candidate (cubic).
+            let mut names = BodyNames::new(st);
             for (k, b) in imp.bodies.iter().enumerate() {
                 let base = f.body_names.get(k).filter(|n| !n.trim().is_empty()).unwrap_or(&b.name);
-                let name = unique_body_name(st, base);
+                let name = names.unique(base);
                 let b = crate::lineage::numbered_tool(b.body.clone().with_import_certificate(&digest, f.id)?);
                 st.bodies.push(ModelBody::traced_new(name, b, f.id));
             }
@@ -4442,6 +4445,35 @@ fn mesh_bodies(meshes: &[crate::MeshData]) -> Result<Vec<(String, Body)>> {
     Ok(out)
 }
 
+/// [`unique_body_name`] for many bodies in a row: the same names, in time linear in the bodies.
+struct BodyNames {
+    taken: std::collections::HashSet<String>,
+    /// The next suffix worth trying for each base (smaller ones are all taken).
+    next: std::collections::HashMap<String, usize>,
+}
+
+impl BodyNames {
+    fn new(st: &ModelState) -> Self {
+        BodyNames { taken: st.bodies.iter().map(|b| b.name.clone()).collect(), next: std::collections::HashMap::new() }
+    }
+    fn unique(&mut self, base: &str) -> String {
+        let base = if base.trim().is_empty() { "Body" } else { base.trim() };
+        if self.taken.insert(base.to_string()) {
+            return base.to_string();
+        }
+        let mut i = self.next.get(base).copied().unwrap_or(2);
+        let name = loop {
+            let candidate = format!("{base} ({i})");
+            i += 1;
+            if self.taken.insert(candidate.clone()) {
+                break candidate;
+            }
+        };
+        self.next.insert(base.to_string(), i);
+        name
+    }
+}
+
 /// `base`, or `base (2)`, `base (3)`… if a body already has that name.
 fn unique_body_name(st: &ModelState, base: &str) -> String {
     let base = if base.trim().is_empty() { "Body" } else { base.trim() };
@@ -4780,3 +4812,33 @@ mod chain_tests {
         assert_eq!(faces.len(), 3, "{faces:?}");
     }
 }
+
+#[cfg(test)]
+mod body_name_tests {
+    use super::*;
+
+    /// Imported bodies get the names one-at-a-time naming gives them, in time linear in the
+    /// bodies: a board file with thousands of same-named parts once took minutes (#883).
+    #[test]
+    fn many_same_named_bodies_are_named_in_linear_time() {
+        let mut st = ModelState::default();
+        let body = kernel::box_solid(Vec3::ZERO, Vec3::new(1.0, 1.0, 1.0)).unwrap();
+        for name in ["R", "R (3)", "C"] {
+            st.bodies.push(ModelBody::new(name.into(), body.clone(), 1));
+        }
+        // The same names as asking the bodies one at a time.
+        let mut names = BodyNames::new(&st);
+        let mut slow = st.clone();
+        for base in ["R", "R", "C", " ", "Body", "R", "R (2)", "X"] {
+            let expected = unique_body_name(&slow, base);
+            assert_eq!(names.unique(base), expected);
+            slow.bodies.push(ModelBody::new(expected, body.clone(), 2));
+        }
+        // 50,000 parts named alike: milliseconds (one-at-a-time naming would take hours).
+        let began = std::time::Instant::now();
+        let mut names = BodyNames::new(&ModelState::default());
+        let last = (0..50_000).map(|_| names.unique("R_0603")).last().unwrap();
+        assert_eq!(last, "R_0603 (50000)");
+        assert!(began.elapsed() < std::time::Duration::from_secs(5), "{:?}", began.elapsed());
+    }
+}
diff --git a/crates/doc/src/progress.rs b/crates/doc/src/progress.rs
index 568d5742..ae063976 100644
--- a/crates/doc/src/progress.rs
+++ b/crates/doc/src/progress.rs
@@ -17,11 +17,23 @@ pub struct Progress {
     total: AtomicUsize,
     /// The feature the evaluation waits at until cancelled ([`Progress::hold_at`]).
     hold: AtomicUsize,
+    /// Long work inside one feature (reading a large STEP file): its step and how far it is.
+    work_stage: std::sync::Mutex<&'static str>,
+    work_done: AtomicUsize,
+    work_total: AtomicUsize,
 }
 
 impl Default for Progress {
     fn default() -> Self {
-        Progress { cancelled: AtomicBool::new(false), done: AtomicUsize::new(0), total: AtomicUsize::new(0), hold: AtomicUsize::new(usize::MAX) }
+        Progress {
+            cancelled: AtomicBool::new(false),
+            done: AtomicUsize::new(0),
+            total: AtomicUsize::new(0),
+            hold: AtomicUsize::new(usize::MAX),
+            work_stage: std::sync::Mutex::new(""),
+            work_done: AtomicUsize::new(0),
+            work_total: AtomicUsize::new(0),
+        }
     }
 }
 
@@ -42,6 +54,29 @@ impl Progress {
     pub fn features(&self) -> (usize, usize) {
         (self.done.load(Ordering::Relaxed), self.total.load(Ordering::Relaxed))
     }
+    /// Long work inside a feature, such as reading a large STEP file: (step, done, total);
+    /// `None` while there is none to show.
+    pub fn work(&self) -> Option<(&'static str, usize, usize)> {
+        let total = self.work_total.load(Ordering::Relaxed);
+        let stage = self.work_stage.lock().map(|s| *s).unwrap_or("");
+        (total > 0 && !stage.is_empty()).then(|| (stage, self.work_done.load(Ordering::Relaxed).min(total), total))
+    }
+}
+
+/// The kernel's long work (STEP import) reports to the same progress and stops on Cancel.
+struct KernelWatch(Arc<Progress>);
+
+impl solidcraft_kernel::watch::Watch for KernelWatch {
+    fn cancelled(&self) -> bool {
+        self.0.is_cancelled()
+    }
+    fn progress(&self, stage: &'static str, done: usize, total: usize) {
+        if let Ok(mut s) = self.0.work_stage.lock() {
+            *s = stage;
+        }
+        self.0.work_total.store(total, Ordering::Relaxed);
+        self.0.work_done.store(done, Ordering::Relaxed);
+    }
 }
 
 struct Current {
@@ -56,6 +91,7 @@ thread_local! {
 
 /// Run `f` with `progress` watching every evaluation it makes on this thread.
 pub fn watch<R>(progress: Arc<Progress>, f: impl FnOnce() -> R) -> R {
+    let kernel = Arc::new(KernelWatch(progress.clone()));
     let previous = CURRENT.with(|c| c.replace(Some(Current { progress, depth: 0 })));
     struct Restore(Option<Option<Current>>);
     impl Drop for Restore {
@@ -66,7 +102,7 @@ pub fn watch<R>(progress: Arc<Progress>, f: impl FnOnce() -> R) -> R {
         }
     }
     let _restore = Restore(Some(previous));
-    f()
+    solidcraft_kernel::watch::watching(kernel, f)
 }
 
 /// Entered at the start of an evaluation; the guard leaves it.
diff --git a/crates/engine/src/cmd/component_clone.rs b/crates/engine/src/cmd/component_clone.rs
index c5895f4e..a52f831c 100644
--- a/crates/engine/src/cmd/component_clone.rs
+++ b/crates/engine/src/cmd/component_clone.rs
@@ -319,7 +319,7 @@ pub(super) fn paste(
         bodies.insert(body.name.clone(), new_name.clone());
         let feature_name = unique(&format!("{} Base", body.name), &mut used);
         let id = scratch.doc_mut().add_feature(
-            FeatureKind::Import { file: format!("{} independent base", body.name), step, components: Vec::new() },
+            FeatureKind::Import { file: format!("{} independent base", body.name), step: step.into(), components: Vec::new() },
             Some(&feature_name),
         )?;
         if let Some(f) = scratch.doc_mut().feature_mut(id) {
diff --git a/crates/engine/src/cmd/config_certificate_tests.rs b/crates/engine/src/cmd/config_certificate_tests.rs
index ec0eceb6..75f850a3 100644
--- a/crates/engine/src/cmd/config_certificate_tests.rs
+++ b/crates/engine/src/cmd/config_certificate_tests.rs
@@ -71,7 +71,10 @@ fn certified_annulus_side_rows_follow_radius_edits_and_clone() {
 
 fn imported(session: &mut Session, step: String, name: &str) -> u64 {
     let owner = session.active_component;
-    let id = session.doc_mut().add_feature(FeatureKind::Import { file: "fixture.step".into(), step, components: vec![] }, Some(name)).unwrap();
+    let id = session
+        .doc_mut()
+        .add_feature(FeatureKind::Import { file: "fixture.step".into(), step: step.into(), components: vec![] }, Some(name))
+        .unwrap();
     let feature = session.doc_mut().feature_mut(id).unwrap();
     feature.body_names = vec![name.into()];
     feature.component = owner;
@@ -262,8 +265,8 @@ fn certified_import_payload_binding_roundtrip_clone_and_replacement_refusal() {
     assert_eq!(painted_radius(&s, "Imported", 10.), pieces);
     // Identical geometry, entity IDs and item IDs, but a replaced embedded payload.
     let FeatureKind::Import { step: payload, .. } = &mut s.doc_mut().feature_mut(source).unwrap().kind else { panic!() };
-    *payload = step.replace("certificate fixture", "replacement fixture");
-    assert_ne!(*payload, step);
+    *payload = step.replace("certificate fixture", "replacement fixture").into();
+    assert_ne!(&**payload, step.as_str());
     s.refresh();
     assert!(s.doc.face_colors(s.model.state().body("Imported").unwrap()).is_empty());
     assert_eq!(painted_radius(&s, "Imported (copy)", 10.), pieces);
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index ef56c232..ef284aa0 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -254,6 +254,7 @@ fn join_of_two_overlapping_rings_is_not_refused() {
 
 mod boolean_diagnostics;
 mod component_frames;
+mod large_step_import;
 mod loft_cut;
 mod mirror_planes;
 mod path_scope;
diff --git a/crates/engine/src/tests/large_step_import.rs b/crates/engine/src/tests/large_step_import.rs
new file mode 100644
index 00000000..58122ea6
--- /dev/null
+++ b/crates/engine/src/tests/large_step_import.rs
@@ -0,0 +1,80 @@
+//! Large STEP imports (#883): the file's text is held once however many undo states share
+//! it, and an import cancelled from another thread stops and keeps nothing.
+
+use std::sync::Arc;
+
+use serde_json::json;
+use solidcraft_doc::{FeatureKind, progress::Progress};
+
+use crate::*;
+
+fn step_file(name: &str, count: usize) -> String {
+    let bodies: Vec<_> = (0..count)
+        .map(|i| {
+            let at = solidcraft_geom::Vec3::new((i % 32) as f64 * 12.0, (i / 32) as f64 * 12.0, 0.0);
+            solidcraft_kernel::box_solid(at, at + solidcraft_geom::Vec3::new(10.0, 10.0, 10.0)).unwrap()
+        })
+        .collect();
+    let text = solidcraft_kernel::step_export(&bodies.iter().collect::<Vec<_>>(), name).unwrap();
+    let path = std::env::temp_dir().join(format!("solidcraft-883-{}-{name}.step", std::process::id()));
+    std::fs::write(&path, text).unwrap();
+    path.to_string_lossy().to_string()
+}
+
+fn step_text(s: &Session) -> Arc<str> {
+    s.doc.features.iter().find_map(|f| if let FeatureKind::Import { step, .. } = &f.kind { Some(step.clone()) } else { None }).unwrap()
+}
+
+/// Memory budget: 30 edits after a large import keep one copy of the STEP text, not 31.
+#[test]
+fn edits_after_a_large_import_share_one_copy_of_its_text() {
+    let path = step_file("shared", 300);
+    let mut s = Session::default();
+    s.execute("file.insert_step", &json!({"path": path})).unwrap();
+    let text = step_text(&s);
+    assert!(text.len() > 500_000, "{} bytes", text.len());
+    for i in 0..30 {
+        s.execute("parameters.add", &json!({"name": format!("p{i}"), "expression": format!("{i} mm")})).unwrap();
+    }
+    assert!(s.undo.len() >= 30);
+    for snap in &s.undo {
+        for f in &snap.doc.features {
+            if let FeatureKind::Import { step, .. } = &f.kind {
+                assert!(Arc::ptr_eq(step, &text), "an undo state holds its own copy of the STEP text");
+            }
+        }
+    }
+    assert!(Arc::ptr_eq(&step_text(&s), &text));
+    // Every Document clone (preview, scratch session) shares it too.
+    let copy = (*s.doc).clone();
+    assert!(copy.features.iter().any(|f| matches!(&f.kind, FeatureKind::Import { step, .. } if Arc::ptr_eq(step, &text))));
+    let _ = std::fs::remove_file(path);
+}
+
+#[test]
+fn cancelled_import_stops_and_leaves_the_design_alone() {
+    let path = step_file("cancelled", 300);
+    let mut s = Session::default();
+    s.execute("solid.box", &json!({"length": 10, "width": 10, "height": 10})).unwrap();
+    let before = s.model.state().bodies.len();
+    // Cancel from the window thread while the worker reads the file.
+    let progress = Arc::new(Progress::default());
+    let canceller = {
+        let progress = progress.clone();
+        std::thread::spawn(move || {
+            while progress.work().is_none() {
+                std::thread::yield_now();
+            }
+            progress.cancel();
+        })
+    };
+    let r = solidcraft_doc::progress::watch(progress.clone(), || s.execute("file.insert_step", &json!({"path": path})));
+    canceller.join().unwrap();
+    let err = r.unwrap_err().to_string();
+    assert!(err.contains("cancelled"), "{err}");
+    assert_eq!(s.model.state().bodies.len(), before);
+    assert!(!s.doc.features.iter().any(|f| matches!(f.kind, FeatureKind::Import { .. })));
+    let (_, done, total) = progress.work().unwrap();
+    assert!(done < total, "stopped part way ({done} of {total})");
+    let _ = std::fs::remove_file(path);
+}
diff --git a/crates/io/src/design.rs b/crates/io/src/design.rs
index 5436e022..e04476ff 100644
--- a/crates/io/src/design.rs
+++ b/crates/io/src/design.rs
@@ -568,7 +568,7 @@ fn read_document<R: Read + Seek>(reader: &mut PackageReader<R>) -> Result<Docume
                 if !step.is_empty() {
                     return Err(invalid("ambiguous embedded STEP and blob"));
                 }
-                *step = std::str::from_utf8(reader.blob(value, "step_blob", "step")?).map_err(invalid)?.to_owned();
+                *step = std::str::from_utf8(reader.blob(value, "step_blob", "step")?).map_err(invalid)?.into();
             }
             FeatureKind::MeshImport { meshes, .. } => {
                 for (index, mesh) in meshes.iter_mut().enumerate() {
@@ -640,7 +640,7 @@ mod tests {
     fn seekable_preview_defers_large_source_blob_until_document_read() {
         let mut doc = Document::new("Deferred source");
         let step = "ISO-10303-21;".repeat(200_000);
-        doc.add_feature(FeatureKind::Import { file: "large.step".into(), step: step.clone(), components: Vec::new() }, None).unwrap();
+        doc.add_feature(FeatureKind::Import { file: "large.step".into(), step: step.clone().into(), components: Vec::new() }, None).unwrap();
         let preview = DesignPreview { meshes: vec![mesh()] };
         let bytes = write_design_container_with_preview(&doc, DesignCompression::Fast, Some(&preview)).unwrap();
         let count = std::rc::Rc::new(std::cell::Cell::new(0));
@@ -699,7 +699,7 @@ mod tests {
     fn native_cache_round_trip_and_preview_defer_original_sources() {
         let mut doc = Document::new("Native cache");
         let step = "ISO-10303-21;".repeat(200_000);
-        doc.add_feature(FeatureKind::Import { file: "large.step".into(), step: step.clone(), components: Vec::new() }, None).unwrap();
+        doc.add_feature(FeatureKind::Import { file: "large.step".into(), step: step.clone().into(), components: Vec::new() }, None).unwrap();
         let bodies = native_geometry();
         let bytes = write_design_container_with_cache(&doc, DesignCompression::Fast, None, Some(&bodies)).unwrap();
         let entries = parts(&bytes);
@@ -868,7 +868,7 @@ mod tests {
         png.extend(150u32.to_be_bytes());
         let mut doc = Document::from_json(&document().to_json()).unwrap();
         if let FeatureKind::Import { step, .. } = &mut doc.features[0].kind {
-            *step = "source data".repeat(250_000);
+            *step = "source data".repeat(250_000).into();
         }
         let bytes = write_design_container_with_thumbnail(&doc, DesignCompression::Fast, None, None, Some(&png)).unwrap();
         let count = std::rc::Rc::new(std::cell::Cell::new(0));
@@ -1157,7 +1157,7 @@ mod tests {
             .collect();
         let step = solidcraft_kernel::step_export(&bodies.iter().collect::<Vec<_>>(), "SolidCraft storage benchmark").unwrap();
         let mut doc = Document::new("Generated STEP benchmark");
-        doc.add_feature(FeatureKind::Import { file: "generated.step".into(), step, components: Vec::new() }, None).unwrap();
+        doc.add_feature(FeatureKind::Import { file: "generated.step".into(), step: step.into(), components: Vec::new() }, None).unwrap();
         measure_storage(&doc, "step");
     }
 
diff --git a/crates/io/src/lib.rs b/crates/io/src/lib.rs
index 3a380344..c4d62eb9 100644
--- a/crates/io/src/lib.rs
+++ b/crates/io/src/lib.rs
@@ -95,6 +95,9 @@ pub struct StepFeature {
     /// Body names from the file (solid names, else product names).
     pub body_names: Vec<String>,
     pub warnings: Vec<String>,
+    /// The file as read. Hold it until the feature has been evaluated: the timeline then reuses
+    /// it instead of reading the file again (it is freed with the last holder).
+    pub import: std::sync::Arc<solidcraft_kernel::StepImport>,
 }
 
 /// Read STEP bytes (`file` is the source file name) into an Import base feature. The file is
@@ -125,8 +128,9 @@ pub fn step_import_feature(bytes: &[u8], file: &str) -> Result<StepFeature> {
     Ok(StepFeature {
         name,
         body_names: imp.bodies.iter().map(|b| b.name.clone()).collect(),
-        kind: solidcraft_doc::FeatureKind::Import { file: file_name, step: text, components: imp.tree.clone() },
+        kind: solidcraft_doc::FeatureKind::Import { file: file_name, step: text.into(), components: imp.tree.clone() },
         warnings: iges_warnings.into_iter().chain(imp.warnings.iter().cloned()).collect(),
+        import: imp,
     })
 }
 
diff --git a/crates/kernel/src/lib.rs b/crates/kernel/src/lib.rs
index a3641986..2ecf305b 100644
--- a/crates/kernel/src/lib.rs
+++ b/crates/kernel/src/lib.rs
@@ -93,6 +93,7 @@ mod topo;
 mod tspline;
 #[cfg(test)]
 mod untouched_faces_tests;
+pub mod watch;
 
 pub use blend::{ChamferSide, chamfer, chamfer_sides, fillet, fillet_each, fillet_g2, fillet_max, fillet_max_each};
 pub use blend_corner::fillet_radii;
@@ -119,7 +120,7 @@ pub use prism_measure::certified_prism_equivalent;
 pub use sew::{EdgeSpec, FaceSpec, SurfSpec, sew};
 pub use splitface::{SplitTool, split_body, split_faces, split_faces_with_map};
 pub use step::step_export;
-pub use step_in::{ImportedBody, StepImport, StepNode, step_import, step_import_shared, step_orientation_errors, step_validate};
+pub use step_in::{ImportedBody, StepImport, StepNode, step_cache_alive, step_import, step_import_shared, step_orientation_errors, step_validate};
 pub use step_out::{ExportBody, ExportProduct, StepHeader, step_export_bodies, step_export_products};
 pub use surface_kind::{FaceSurface, face_surface};
 pub use surface_offset::offset_surface;
diff --git a/crates/kernel/src/step_in/mod.rs b/crates/kernel/src/step_in/mod.rs
index 6070e017..53818e6a 100644
--- a/crates/kernel/src/step_in/mod.rs
+++ b/crates/kernel/src/step_in/mod.rs
@@ -257,6 +257,12 @@ struct Reader<'a> {
     unmeshed_checked: HashSet<u64>,
     bodies: Vec<ImportedBody>,
     workers: usize,
+    /// Whoever waits for the import (progress, Cancel).
+    watch: Option<Arc<dyn crate::watch::Watch>>,
+    /// Solid items in the file, the measure of the conversion's progress.
+    solid_items: usize,
+    /// The watch cancelled the import: the walk stops and the import fails.
+    cancelled: bool,
 }
 
 impl<'a> Reader<'a> {
@@ -280,9 +286,27 @@ impl<'a> Reader<'a> {
             unmeshed_checked: HashSet::new(),
             bodies: Vec::new(),
             workers,
+            watch: None,
+            solid_items: 0,
+            cancelled: false,
         }
     }
 
+    /// Report progress; true once the import is cancelled (the walk then unwinds).
+    fn stop(&mut self) -> bool {
+        if self.cancelled {
+            return true;
+        }
+        if let Some(w) = &self.watch {
+            if w.cancelled() {
+                self.cancelled = true;
+                return true;
+            }
+            w.progress("converting", self.bodies.len().min(self.solid_items), self.solid_items);
+        }
+        false
+    }
+
     fn warn(&mut self, msg: impl Into<String>) {
         *self.warnings.entry(msg.into()).or_default() += 1;
     }
@@ -584,6 +608,9 @@ impl<'a> Reader<'a> {
         let u = self.units_of(rep);
         let mut items = self.reps.get(&rep).map(|r| r.0.clone()).unwrap_or_default().into_iter().peekable();
         while let Some(item) = items.next() {
+            if self.stop() {
+                return;
+            }
             let Some(e) = self.ex.get(item) else { continue };
             match e.name() {
                 "MANIFOLD_SOLID_BREP" | "BREP_WITH_VOIDS" | "FACETED_BREP" | "SHELL_BASED_SURFACE_MODEL" => {
@@ -749,6 +776,9 @@ impl<'a> Reader<'a> {
     fn node(&mut self, pd: u64, local: mt::Matrix4, world: mt::Matrix4, path: &[String], depth: usize, stack: &mut Vec<u64>) -> StepNode {
         let name = self.product_name(pd);
         let mut node = StepNode { name: name.clone(), transform: to_array(&local), bodies: Vec::new(), children: Vec::new() };
+        if self.stop() {
+            return node;
+        }
         if depth > MAX_ASSEMBLY_DEPTH || stack.contains(&pd) {
             self.warn("assembly nested too deeply or recursive: some components were left out");
             return node;
@@ -933,40 +963,73 @@ pub fn step_validate(text: &str) -> std::result::Result<usize, String> {
     Ok(n)
 }
 
+/// A recent import: held weakly, so it lives only as long as someone uses it (the command that
+/// read the file, until the timeline has evaluated it), and strongly only when it is small.
+struct Recent {
+    key: (u64, usize),
+    import: std::sync::Weak<StepImport>,
+    /// Keeps a small import alive (never read).
+    _small: Option<Arc<StepImport>>,
+}
+
 /// Recent imports, so a file read for a command and then evaluated in the timeline is only
 /// read once.
-static RECENT: std::sync::Mutex<Vec<(u64, usize, std::sync::Arc<StepImport>)>> = std::sync::Mutex::new(Vec::new());
+static RECENT: std::sync::Mutex<Vec<Recent>> = std::sync::Mutex::new(Vec::new());
 
-/// [`step_import`] through a small cache of recent results (keyed by the text).
+/// Files up to this size stay cached after their last user is gone (a few cheap re-reads
+/// saved); a larger import is freed with the design that holds it.
+const KEEP_SMALL_BYTES: usize = 1 << 20;
+
+/// [`step_import`] through a small cache of recent results (keyed by the text). Keep the result
+/// while the same text is still to be read elsewhere (in the timeline): that read reuses it.
 pub fn step_import_shared(text: &str) -> Result<std::sync::Arc<StepImport>> {
     use std::hash::{Hash, Hasher};
     let mut h = std::collections::hash_map::DefaultHasher::new();
     text.hash(&mut h);
     let key = (h.finish(), text.len());
     if let Ok(cache) = RECENT.lock()
-        && let Some((_, _, imp)) = cache.iter().find(|(k, n, _)| (*k, *n) == key)
+        && let Some(imp) = cache.iter().filter(|r| r.key == key).find_map(|r| r.import.upgrade())
     {
-        return Ok(imp.clone());
+        return Ok(imp);
     }
     let imp = std::sync::Arc::new(step_import(text)?);
     if let Ok(mut cache) = RECENT.lock() {
-        cache.retain(|(k, n, _)| (*k, *n) != key);
-        cache.push((key.0, key.1, imp.clone()));
-        if cache.len() > 4 {
+        cache.retain(|r| r.key != key && r.import.strong_count() > 0);
+        let small = (text.len() <= KEEP_SMALL_BYTES).then(|| imp.clone());
+        cache.push(Recent { key, import: Arc::downgrade(&imp), _small: small });
+        while cache.len() > 4 {
             cache.remove(0);
         }
     }
     Ok(imp)
 }
 
+/// Imports the cache still holds alive, and how many STEP bytes they came from (tests and
+/// diagnostics: a closed design must not leave its import behind).
+pub fn step_cache_alive() -> (usize, usize) {
+    RECENT.lock().map(|c| c.iter().filter(|r| r.import.strong_count() > 0).fold((0, 0), |(n, b), r| (n + 1, b + r.key.1))).unwrap_or((0, 0))
+}
+
 /// Read a STEP file: its solids (as bodies, placed by the assembly structure) and the tree.
 pub fn step_import(text: &str) -> Result<StepImport> {
     step_import_with_workers(text, parallel::workers())
 }
 
 fn step_import_with_workers(text: &str, workers: usize) -> Result<StepImport> {
-    let ex = p21::parse(text).map_err(KernelError::Invalid)?;
+    let watch = crate::watch::current();
+    let cancelled = || watch.as_ref().is_some_and(|w| w.cancelled());
+    let ex = match p21::parse(text) {
+        Ok(ex) => ex,
+        Err(_) if cancelled() => return Err(crate::watch::cancelled_error("STEP import")),
+        Err(e) => return Err(KernelError::Invalid(e)),
+    };
     let mut rd = Reader::new(&ex, workers);
+    rd.solid_items = ex
+        .entities
+        .values()
+        .filter(|e| matches!(e.name(), "MANIFOLD_SOLID_BREP" | "BREP_WITH_VOIDS" | "FACETED_BREP" | "SHELL_BASED_SURFACE_MODEL"))
+        .count();
+    rd.watch = watch.clone();
     rd.index();
     // Roots: product definitions that are nobody's component.
     let child_pds: HashSet<u64> =
@@ -1006,6 +1069,10 @@ fn step_import_with_workers(text: &str, workers: usize) -> Result<StepImport> {
             tree.push(StepNode { name: "Import".into(), transform: to_array(&identity), bodies, children: Vec::new() });
         }
     }
+    // Cancelled: what was read so far is dropped here, not cached.
+    if rd.cancelled || cancelled() {
+        return Err(crate::watch::cancelled_error("STEP import"));
+    }
     if let Some(first) = ex.duplicates.first() {
         rd.warn(format!("{} entity ids are defined twice (first #{first}); the first definitions were used", ex.duplicates.len()));
     }
diff --git a/crates/kernel/src/step_in/p21.rs b/crates/kernel/src/step_in/p21.rs
index 996583cb..e749f40b 100644
--- a/crates/kernel/src/step_in/p21.rs
+++ b/crates/kernel/src/step_in/p21.rs
@@ -399,6 +399,7 @@ pub fn parse(text: &str) -> Result<Exchange, String> {
     }
     lx.expect(b';')?;
     let mut ex = Exchange::default();
+    let watch = crate::watch::current();
     loop {
         lx.ws()?;
         if lx.peek().is_none() {
@@ -469,6 +470,14 @@ pub fn parse(text: &str) -> Result<Exchange, String> {
                         vec![lx.record()?]
                     };
                     lx.expect(b';')?;
+                    if ex.entities.len() % 4096 == 0
+                        && let Some(w) = &watch
+                    {
+                        if w.cancelled() {
+                            return err("STEP import cancelled");
+                        }
+                        w.progress("reading", lx.i, lx.s.len());
+                    }
                     if ex.entities.len() >= MAX_ENTITIES {
                         return err(format!("STEP file has more than {MAX_ENTITIES} entities"));
                     }
diff --git a/crates/kernel/src/step_in/tests.rs b/crates/kernel/src/step_in/tests.rs
index f943651a..6932be2a 100644
--- a/crates/kernel/src/step_in/tests.rs
+++ b/crates/kernel/src/step_in/tests.rs
@@ -915,3 +915,119 @@ fn faces_with_too_many_bounds_are_refused() {
     assert!(imp.warnings.iter().any(|w| w.contains("boundary loops")), "{:?}", imp.warnings);
     assert!(start.elapsed() < std::time::Duration::from_secs(10), "took {:?}", start.elapsed());
 }
+
+/// A board-like assembly for issue #883: `SOLIDCRAFT_STEP_PCB_FIXTURE=path` writes it (with
+/// `SOLIDCRAFT_STEP_PCB_COMPONENTS` and `SOLIDCRAFT_STEP_PCB_PINS`; 600 and 15 give 52 MB). Every component is
+/// its own product with a package body and its own pins (nothing shared), the worst case for
+/// a large import.
+#[test]
+#[ignore = "manual fixture generator for large STEP imports"]
+fn generate_board_assembly_fixture() {
+    use crate::step_out::{ExportBody, ExportProduct, StepHeader, step_export_products};
+    let count: usize = solidcraft_geom::env::var("STEP_PCB_COMPONENTS").map(|s| s.parse().unwrap()).unwrap_or(500);
+    let pins: usize = solidcraft_geom::env::var("STEP_PCB_PINS").map(|s| s.parse().unwrap()).unwrap_or(16);
+    let mut owned: Vec<Vec<Body>> = Vec::new();
+    for c in 0..count {
+        let w = 4.0 + (c % 7) as f64 * 0.37;
+        let d = 3.0 + (c % 5) as f64 * 0.29;
+        let mut bodies = vec![fillet_box(w, d, 1.2 + (c % 3) as f64 * 0.1)];
+        for p in 0..pins {
+            let side = if p % 2 == 0 { -0.6 } else { d + 0.6 };
+            let x = 0.3 + (p / 2) as f64 * (w - 0.6) / (pins / 2).max(1) as f64 + c as f64 * 1e-4;
+            bodies.push(cylinder(Vec3::new(x, side, -1.5), Vec3::Z, 0.25, 2.0 + (p % 3) as f64 * 0.01).unwrap());
+        }
+        owned.push(bodies);
+    }
+    let mut products: Vec<ExportProduct> = owned
+        .iter()
+        .enumerate()
+        .map(|(c, bodies)| ExportProduct {
+            name: format!("U{c}"),
+            bodies: bodies.iter().enumerate().map(|(k, b)| ExportBody { name: format!("U{c}_{k}"), body: b, color: Some([0.2, 0.2, 0.2]) }).collect(),
+            children: Vec::new(),
+        })
+        .collect();
+    let side = (count as f64).sqrt().ceil() as usize;
+    let children = (0..count)
+        .map(|c| {
+            let (x, y) = ((c % side) as f64 * 12.0, (c / side) as f64 * 10.0);
+            (c, [[1.0, 0.0, 0.0, 0.0], [0.0, 1.0, 0.0, 0.0], [0.0, 0.0, 1.0, 0.0], [x, y, 1.6, 1.0]], format!("U{c}"))
+        })
+        .collect();
+    products.push(ExportProduct { name: "Board".into(), bodies: Vec::new(), children });
+    let root = products.len() - 1;
+    let text = step_export_products(&products, root, &StepHeader { file_name: "board.step".into(), ..Default::default() }).unwrap();
+    let path = solidcraft_geom::env::var("STEP_PCB_FIXTURE").unwrap_or_else(|| "board.step".into());
+    std::fs::write(&path, &text).unwrap();
+    println!("wrote {path}: {} bytes, {} components", text.len(), count);
+}
+
+fn fillet_box(w: f64, d: f64, h: f64) -> Body {
+    let b = box_solid(Vec3::ZERO, Vec3::new(w, d, h)).unwrap();
+    let vertical = [Vec3::new(0.0, 0.0, h / 2.0), Vec3::new(w, 0.0, h / 2.0), Vec3::new(w, d, h / 2.0), Vec3::new(0.0, d, h / 2.0)];
+    let top = [Vec3::new(w / 2.0, 0.0, h), Vec3::new(w, d / 2.0, h), Vec3::new(w / 2.0, d, h), Vec3::new(0.0, d / 2.0, h)];
+    let b = fillet(&b, &vertical, 0.3).unwrap_or(b);
+    fillet(&b, &top, 0.1).unwrap_or(b)
+}
+
+/// Independent boxes as one STEP file (about 3 KB a box).
+fn boxes_step(count: usize, name: &str) -> String {
+    let bodies: Vec<_> = (0..count)
+        .map(|i| {
+            let at = Vec3::new((i % 32) as f64 * 12.0, (i / 32) as f64 * 12.0, 0.0);
+            box_solid(at, at + Vec3::new(10.0, 10.0, 10.0)).unwrap()
+        })
+        .collect();
+    step_export(&bodies.iter().collect::<Vec<_>>(), name).unwrap()
+}
+
+struct TestWatch {
+    cancel_after: usize,
+    reports: std::sync::atomic::AtomicUsize,
+    last: std::sync::Mutex<(&'static str, usize, usize)>,
+}
+
+impl crate::watch::Watch for TestWatch {
+    fn cancelled(&self) -> bool {
+        self.reports.load(std::sync::atomic::Ordering::Relaxed) >= self.cancel_after
+    }
+    fn progress(&self, stage: &'static str, done: usize, total: usize) {
+        self.reports.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+        *self.last.lock().unwrap() = (stage, done, total);
+    }
+}
+
+#[test]
+fn watched_import_reports_progress_and_stops_on_cancel_without_caching() {
+    let text = boxes_step(40, "watched import");
+    let watch = Arc::new(TestWatch { cancel_after: usize::MAX, reports: Default::default(), last: std::sync::Mutex::new(("", 0, 0)) });
+    let imp = crate::watch::watching(watch.clone(), || step_import(&text)).unwrap();
+    assert_eq!(imp.bodies.len(), 40);
+    let (stage, done, total) = *watch.last.lock().unwrap();
+    assert_eq!((stage, total), ("converting", 40));
+    assert!(done <= total && watch.reports.load(std::sync::atomic::Ordering::Relaxed) > 1);
+
+    // Cancelled part way: an error, no partial result, and nothing cached for the text.
+    let text = boxes_step(41, "cancelled import");
+    let watch = Arc::new(TestWatch { cancel_after: 3, reports: Default::default(), last: std::sync::Mutex::new(("", 0, 0)) });
+    let err = crate::watch::watching(watch.clone(), || step_import_shared(&text).map(|_| ())).unwrap_err();
+    assert!(err.to_string().contains("cancelled"), "{err}");
+    let (_, done, _) = *watch.last.lock().unwrap();
+    assert!(done < 41, "stopped before converting everything ({done})");
+    // Unwatched, the same text imports in full (no cached failure or partial import).
+    assert_eq!(step_import_shared(&text).unwrap().bodies.len(), 41);
+    // Without a watch nothing is reported or stopped.
+    assert_eq!(step_import(&boxes_step(2, "unwatched")).unwrap().bodies.len(), 2);
+}
+
+#[test]
+fn large_imports_are_freed_with_their_last_user() {
+    let text = boxes_step(400, "large shared import");
+    assert!(text.len() > KEEP_SMALL_BYTES, "{} bytes", text.len());
+    let first = step_import_shared(&text).unwrap();
+    let again = step_import_shared(&text).unwrap();
+    assert!(Arc::ptr_eq(&first, &again), "read once while it is in use");
+    let weak = Arc::downgrade(&first);
+    drop((first, again));
+    assert!(weak.upgrade().is_none(), "the cache must not keep a large import alive after its design is gone");
+}
diff --git a/crates/kernel/src/watch.rs b/crates/kernel/src/watch.rs
new file mode 100644
index 00000000..840988c2
--- /dev/null
+++ b/crates/kernel/src/watch.rs
@@ -0,0 +1,44 @@
+//! Progress and cooperative cancellation for long kernel work (reading a large STEP file) run
+//! off the window thread. The caller installs a [`Watch`] around the work with [`watching`];
+//! the work reports how far it is and stops at its next check once the watch says so. Work
+//! that nobody watches runs as before.
+
+use std::cell::RefCell;
+use std::sync::Arc;
+
+/// Someone waiting for long work: told how far it is, and able to stop it.
+pub trait Watch: Send + Sync {
+    /// Stop at the next check.
+    fn cancelled(&self) -> bool;
+    /// `done` of `total` units of the current step (`stage`, such as "reading" or "converting").
+    fn progress(&self, stage: &'static str, done: usize, total: usize);
+}
+
+thread_local! {
+    static CURRENT: RefCell<Option<Arc<dyn Watch>>> = const { RefCell::new(None) };
+}
+
+/// Run `f` with `watch` seeing the kernel work it does on this thread.
+pub fn watching<R>(watch: Arc<dyn Watch>, f: impl FnOnce() -> R) -> R {
+    let previous = CURRENT.with(|c| c.replace(Some(watch)));
+    struct Restore(Option<Option<Arc<dyn Watch>>>);
+    impl Drop for Restore {
+        fn drop(&mut self) {
+            if let Some(previous) = self.0.take() {
+                CURRENT.with(|c| *c.borrow_mut() = previous);
+            }
+        }
+    }
+    let _restore = Restore(Some(previous));
+    f()
+}
+
+/// The watch over this thread's work, to hand to the worker threads it starts.
+pub(crate) fn current() -> Option<Arc<dyn Watch>> {
+    CURRENT.with(|c| c.borrow().clone())
+}
+
+/// The error long work returns when its watch cancelled it.
+pub(crate) fn cancelled_error(what: &str) -> crate::KernelError {
+    crate::KernelError::Failed(format!("{what} cancelled"))
+}
diff --git a/crates/ui-egui/src/file_load.rs b/crates/ui-egui/src/file_load.rs
index 4e4b5eb7..1a37924b 100644
--- a/crates/ui-egui/src/file_load.rs
+++ b/crates/ui-egui/src/file_load.rs
@@ -474,6 +474,8 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
     };
     let phase = r.phase.load(Ordering::Relaxed);
     let (done, total) = r.progress.features();
+    // Long work inside one feature (a large STEP file) has progress of its own.
+    let work = r.progress.work().filter(|(_, d, t)| phase == 0 && d < t);
     let screen = ctx.content_rect();
     // Leave the title bar to the window. The resize edges are a foreground layer above this.
     let cover = egui::Rect::from_min_max(egui::pos2(screen.left(), screen.top() + crate::toolbar::APP_BAR_HEIGHT), screen.max);
@@ -538,16 +540,22 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                 ui.label(if cancelled {
                     "Waiting for the previous operation to stop...".to_string()
                 } else {
-                    match phase {
-                        1 => "Preparing the view...".into(),
-                        2 => "Preparing the thumbnail...".into(),
+                    match (phase, work) {
+                        (_, Some(("reading", d, t))) => format!("Reading the file... {}%", d * 100 / t.max(1)),
+                        (_, Some((_, d, t))) => format!("Converting solid {} of {t}", d + 1),
+                        (1, _) => "Preparing the view...".into(),
+                        (2, _) => "Preparing the thumbnail...".into(),
                         _ if total > 0 => format!("Rebuilding feature {} of {total}", (done + 1).min(total)),
                         _ => "Reading the file...".into(),
                     }
                 });
             });
             // Always there, so the card keeps its height as the phases change.
-            let fraction = if cancelled || total == 0 {
+            let fraction = if cancelled {
+                0.0
+            } else if let Some((_, d, t)) = work {
+                d as f32 / t.max(1) as f32
+            } else if total == 0 {
                 0.0
             } else if phase == 0 {
                 done as f32 / total as f32
diff --git a/crates/ui-egui/src/gpu.rs b/crates/ui-egui/src/gpu.rs
index 62fa2935..af854ea0 100644
--- a/crates/ui-egui/src/gpu.rs
+++ b/crates/ui-egui/src/gpu.rs
@@ -179,9 +179,13 @@ pub struct AccessMap {
     pub scale: f32,
 }
 
+/// Vertices (or line instances) on the GPU, in as many buffers as the device's buffer size
+/// limit needs: a large import's triangles can exceed one buffer (#883).
 struct Batch {
-    buffer: wgpu::Buffer,
-    count: u32,
+    /// Each buffer and its vertex (instance) count.
+    parts: Vec<(wgpu::Buffer, u32)>,
+    /// Bytes per buffer (all but the last are full).
+    chunk: usize,
 }
 
 /// A chunk of planned bodies on the GPU: their evaluated mesh and a colour per vertex.
@@ -289,8 +293,8 @@ impl Batches {
                 let faces = mesh.vertex_face.get(part.vertices.start as usize..part.vertices.end as usize).unwrap_or_default();
                 bytes.extend(faces.iter().flat_map(|f| b.face_colors.get(*f as usize).copied().unwrap_or(last)));
             }
-            if let Some(colors) = upload(device, "sc_planned_colors", &bytes, 4) {
-                out.push(PlannedDraw { mesh, colors: colors.buffer });
+            if let Some((colors, _)) = upload(device, "sc_planned_colors", &bytes, 4).and_then(|c| c.parts.into_iter().next()) {
+                out.push(PlannedDraw { mesh, colors });
             }
         }
         out
@@ -663,13 +667,27 @@ fn upload(device: &wgpu::Device, label: &str, bytes: &[u8], stride: usize) -> Op
 }
 
 fn upload_with(device: &wgpu::Device, label: &str, bytes: &[u8], stride: usize, usage: wgpu::BufferUsages) -> Option<Batch> {
-    if bytes.len() < stride {
+    if bytes.len() < stride || stride == 0 {
         return None;
     }
-    Some(Batch {
-        buffer: device.create_buffer_init(&wgpu::util::BufferInitDescriptor { label: Some(label), contents: bytes, usage }),
-        count: u32::try_from(bytes.len() / stride).unwrap_or(0),
-    })
+    let chunk = buffer_chunk(stride, device.limits().max_buffer_size);
+    let parts = bytes
+        .chunks(chunk)
+        .filter(|part| part.len() >= stride)
+        .map(|part| {
+            let buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor { label: Some(label), contents: part, usage });
+            (buffer, u32::try_from(part.len() / stride).unwrap_or(0))
+        })
+        .collect();
+    Some(Batch { parts, chunk })
+}
+
+/// Bytes per GPU buffer for vertices of `stride` bytes: within the device's limit and a
+/// whole number of triangles and line pairs, so no primitive straddles two buffers.
+fn buffer_chunk(stride: usize, max_buffer_size: u64) -> usize {
+    let unit = stride.max(1) * 6;
+    let max = usize::try_from(max_buffer_size).unwrap_or(usize::MAX).min(1 << 30);
+    (max / unit).max(1) * unit
 }
 
 /// Triangles (3 vertices of `TRI_SIZE` bytes each, position first) ordered far to near along
@@ -691,7 +709,10 @@ impl Batches {
         let (Some(g), false) = (&self.glass, self.glass_cpu.is_empty()) else { return };
         let turned = self.glass_sorted_for.is_none_or(|s| s[0] * back[0] + s[1] * back[1] + s[2] * back[2] < 0.9994);
         if turned {
-            queue.write_buffer(&g.buffer, 0, &sort_back_to_front(&self.glass_cpu, back));
+            let sorted = sort_back_to_front(&self.glass_cpu, back);
+            for ((buffer, _), bytes) in g.parts.iter().zip(sorted.chunks(g.chunk.max(1))) {
+                queue.write_buffer(buffer, 0, bytes);
+            }
             self.glass_sorted_for = Some(back);
         }
     }
@@ -796,15 +817,19 @@ impl egui_wgpu::CallbackTrait for ViewportCallback {
         let tris = |pass: &mut wgpu::RenderPass<'static>, p: &wgpu::RenderPipeline, b: &Option<Batch>| {
             if let Some(b) = b {
                 pass.set_pipeline(p);
-                pass.set_vertex_buffer(0, b.buffer.slice(..));
-                pass.draw(0..b.count, 0..1);
+                for (buffer, count) in &b.parts {
+                    pass.set_vertex_buffer(0, buffer.slice(..));
+                    pass.draw(0..*count, 0..1);
+                }
             }
         };
         let lines = |pass: &mut wgpu::RenderPass<'static>, p: &wgpu::RenderPipeline, b: &Option<Batch>| {
             if let Some(b) = b {
                 pass.set_pipeline(p);
-                pass.set_vertex_buffer(0, b.buffer.slice(..));
-                pass.draw(0..6, 0..b.count);
+                for (buffer, count) in &b.parts {
+                    pass.set_vertex_buffer(0, buffer.slice(..));
+                    pass.draw(0..6, 0..*count);
+                }
             }
         };
         let (m, sk, h, pv) = (&res.model, &res.sketch, &res.highlight, &res.preview);
@@ -848,8 +873,10 @@ impl egui_wgpu::CallbackTrait for ViewportCallback {
             if let Some((_, bind)) = res.textures.get(id) {
                 pass.set_pipeline(&res.image);
                 pass.set_bind_group(1, bind, &[]);
-                pass.set_vertex_buffer(0, q.buffer.slice(..));
-                pass.draw(0..q.count, 0..1);
+                for (buffer, count) in &q.parts {
+                    pass.set_vertex_buffer(0, buffer.slice(..));
+                    pass.draw(0..*count, 0..1);
+                }
             }
         }
         tris(pass, &res.ghost, &pv.ghost);
@@ -1079,6 +1106,22 @@ mod tests {
 mod glass_tests {
     use super::*;
 
+    /// A large import's triangles once went into one buffer past the device's limit (524 MB
+    /// against 256 MB) and the app crashed (#883): they are split into whole triangles.
+    #[test]
+    fn large_scenes_are_split_into_buffers_within_the_device_limit() {
+        let limit = 256u64 << 20;
+        for stride in [TRI_SIZE, LINE_SIZE, 4] {
+            let chunk = buffer_chunk(stride, limit);
+            assert!(chunk as u64 <= limit && chunk > limit as usize / 2);
+            assert_eq!(chunk % (stride * 3), 0, "whole triangles");
+            assert_eq!(chunk % (stride * 2), 0, "whole line pairs");
+        }
+        let scene = 524_434_680usize;
+        assert_eq!(scene.div_ceil(buffer_chunk(TRI_SIZE, limit)), 2);
+        assert!(buffer_chunk(TRI_SIZE, 1) >= TRI_SIZE * 6, "a tiny limit still holds one unit");
+    }
+
     #[test]
     fn glass_is_drawn_far_to_near() {
         // Two triangles at z = 5 and z = -5, given near first; looking down from +Z.
diff --git a/crates/ui-egui/src/viewport_appearance_tests.rs b/crates/ui-egui/src/viewport_appearance_tests.rs
index 7f65f589..b7f2fffb 100644
--- a/crates/ui-egui/src/viewport_appearance_tests.rs
+++ b/crates/ui-egui/src/viewport_appearance_tests.rs
@@ -14,7 +14,8 @@ fn placed_step_body_uses_imported_opacity_until_a_design_look_overrides_it() {
             .unwrap();
     let mut document = doc::Document::default();
     let component = document.add_component(Some("Imported part"), 0).unwrap();
-    let feature = document.add_feature(doc::FeatureKind::Import { file: "glass.step".into(), step, components: Vec::new() }, None).unwrap();
+    let feature =
+        document.add_feature(doc::FeatureKind::Import { file: "glass.step".into(), step: step.into(), components: Vec::new() }, None).unwrap();
     document.features.iter_mut().find(|f| f.id == feature).unwrap().component = component;
     let mut session = Session::new(document);
     let source = session.model.state().bodies[0].name.clone();