diff --git a/apps/solidcraft/src/main.rs b/apps/solidcraft/src/main.rs
index 1c6beb3d..ee4162fb 100644
--- a/apps/solidcraft/src/main.rs
+++ b/apps/solidcraft/src/main.rs
@@ -456,6 +456,7 @@ fn main() -> eframe::Result {
             solidcraft_engine::crash::phase("app setup");
             solidcraft_ui_egui::fonts::setup_fonts(&cc.egui_ctx);
             let mut app = SolveApp::new(Session::default(), services(cc));
+            app.apply_off_thread = Some(solidcraft_ui_egui::dialogs::APPLY_AT_ONCE);
             app.custom_titlebar = CUSTOM_TITLEBAR;
             app.integrated_titlebar = cfg!(target_os = "macos");
             app.cpu_meshing = safe_mode;
diff --git a/assets/locales/de.json b/assets/locales/de.json
index e9764a79..bf2eb8ea 100644
--- a/assets/locales/de.json
+++ b/assets/locales/de.json
@@ -10,6 +10,7 @@
   "browser.precision": "Genauigkeit: {precision}",
   "browser.sketch_name": "Skizze {number}",
   "browser.units": "Einheiten: {units}",
+  "dialog.computing": "Wird berechnet… {seconds} s",
   "dialog.custom_axis": "↖ Eigene Achse",
   "dialog.edit_title": "{title} bearbeiten",
   "dialog.select": "↖ Auswählen",
diff --git a/assets/locales/en.json b/assets/locales/en.json
index af0e10c5..7729a6e1 100644
--- a/assets/locales/en.json
+++ b/assets/locales/en.json
@@ -286,6 +286,7 @@
   "cmd.view_list": "List Named Views",
   "cmd.view_rename": "Rename Named View",
   "cmd.view_save": "New Named View",
+  "dialog.computing": "Computing… {seconds} s",
   "dialog.custom_axis": "↖ Custom Axis",
   "dialog.edit_title": "EDIT {title}",
   "dialog.select": "↖ Select",
diff --git a/assets/locales/ja.json b/assets/locales/ja.json
index 0f349f25..f8f17f7d 100644
--- a/assets/locales/ja.json
+++ b/assets/locales/ja.json
@@ -286,6 +286,7 @@
   "cmd.view_list": "名前付きビューを一覧表示",
   "cmd.view_rename": "名前付きビューの名前を変更",
   "cmd.view_save": "新規名前付きビュー",
+  "dialog.computing": "計算中… {seconds} 秒",
   "dialog.custom_axis": "↖ カスタム軸",
   "dialog.edit_title": "{title}を編集",
   "dialog.select": "↖ 選択",
diff --git a/assets/locales/ko.json b/assets/locales/ko.json
index 3b6cf556..2607933e 100644
--- a/assets/locales/ko.json
+++ b/assets/locales/ko.json
@@ -10,6 +10,7 @@
   "browser.precision": "정밀도: {precision}",
   "browser.sketch_name": "스케치{number}",
   "browser.units": "단위: {units}",
+  "dialog.computing": "계산 중… {seconds}초",
   "dialog.custom_axis": "↖ 사용자 정의 축",
   "dialog.edit_title": "{title} 편집",
   "dialog.select": "↖ 선택",
diff --git a/assets/locales/zh.json b/assets/locales/zh.json
index c183f897..7b75f38a 100644
--- a/assets/locales/zh.json
+++ b/assets/locales/zh.json
@@ -286,6 +286,7 @@
   "cmd.view_list": "列出命名视图",
   "cmd.view_rename": "重命名命名视图",
   "cmd.view_save": "新建命名视图",
+  "dialog.computing": "正在计算… {seconds} 秒",
   "dialog.custom_axis": "↖ 自定义轴",
   "dialog.edit_title": "编辑 {title}",
   "dialog.select": "↖ 选择",
diff --git a/crates/ui-egui/src/apply_off_thread_tests.rs b/crates/ui-egui/src/apply_off_thread_tests.rs
new file mode 100644
index 00000000..0cd4c134
--- /dev/null
+++ b/crates/ui-egui/src/apply_off_thread_tests.rs
@@ -0,0 +1,118 @@
+//! OK off the window thread: a feature that takes longer than the app's wait is computed beside
+//! the window, the dialog saying so, and applied once it is in; Cancel drops it.
+
+use std::time::Duration;
+
+use serde_json::json;
+
+use crate::scenario::Harness;
+
+/// A 60 × 20 × 10 block and, just finished on its top, a 10 × 10 square: Extrude takes it.
+fn square_on_a_block() -> Harness {
+    let mut h = Harness::new();
+    h.app.home.open = false;
+    for (id, p) in [
+        ("sketch.create", json!({"plane": "XY"})),
+        ("sketch.rectangle.two_point", json!({"p0": [0, 0], "p1": [60, 20]})),
+        ("sketch.finish", json!({})),
+        ("solid.extrude", json!({"distance": 10})),
+        ("sketch.create", json!({"plane": {"origin": [0, 0, 10], "x_dir": [1, 0, 0], "y_dir": [0, 1, 0]}})),
+        ("sketch.rectangle.two_point", json!({"p0": [5, 5], "p1": [15, 15]})),
+    ] {
+        h.app.run(id, p).unwrap();
+    }
+    h.app.finish_sketch();
+    h.app.start("solid.extrude");
+    h.frames(2);
+    h
+}
+
+/// Frames until OK's evaluation is in and applied (a few seconds at most).
+fn until_applied(h: &mut Harness) {
+    for _ in 0..500 {
+        if h.app.applying.is_none() {
+            break;
+        }
+        h.frames(1);
+        std::thread::sleep(Duration::from_millis(10));
+    }
+    h.frames(1);
+}
+
+#[test]
+fn ok_computes_off_the_window_thread_saying_so_then_applies() {
+    let mut h = square_on_a_block();
+    h.app.apply_off_thread = Some(Duration::ZERO);
+    h.step(&json!({"key": "Enter"})).unwrap();
+    assert!(h.app.applying.is_some() && h.app.dialog.is_some(), "computing, the dialog open");
+    // The evaluation may be in by the next frame: it is held until the dialog has been seen
+    // saying so (the evaluation itself is in the cache all the same).
+    let (finish, done) = std::sync::mpsc::channel();
+    if let Some(job) = h.app.applying.as_mut() {
+        let _ = job.done.recv();
+        job.done = done;
+    }
+    h.frames(1);
+    assert!(h.widget("Computing… 0 s").is_some(), "the dialog says it computes");
+    // Enter again does not start it twice.
+    h.step(&json!({"key": "Enter"})).unwrap();
+    finish.send(()).unwrap();
+    until_applied(&mut h);
+    assert!(h.app.dialog.is_none(), "{:?}", h.app.dialog.as_ref().and_then(|d| d.error.clone()));
+    h.step(&json!({"expect": {"bodies": 1, "volume": 13000}})).unwrap();
+    assert_eq!(h.app.session.doc.features.len(), 4, "one extrude added");
+}
+
+#[test]
+fn cancel_while_computing_applies_nothing() {
+    let mut h = square_on_a_block();
+    h.app.apply_off_thread = Some(Duration::ZERO);
+    let features = h.app.session.doc.features.len();
+    h.step(&json!({"key": "Enter"})).unwrap();
+    assert!(h.app.applying.is_some());
+    h.step(&json!({"key": "Escape"})).unwrap();
+    assert!(h.app.applying.is_none() && h.app.dialog.is_none());
+    h.frames(5);
+    assert_eq!(h.app.session.doc.features.len(), features);
+}
+
+#[test]
+fn the_dialog_holds_its_inputs_while_ok_computes() {
+    let mut h = square_on_a_block();
+    h.app.fit_view();
+    h.frames(2);
+    let picked = h.app.dialog.as_ref().map(|d| d.inputs[0].items.clone()).unwrap();
+    // OK computing, until the test lets it finish.
+    let (finish, done) = std::sync::mpsc::channel();
+    let cmds = h.app.dialog.as_ref().and_then(|d| crate::dialogs::apply_commands(&h.app, d).ok()).unwrap();
+    h.app.applying = Some(crate::dialogs::Applying { done, started: crate::now_ms(), cmds });
+    // A click on the square picked would take it out of the profiles.
+    h.step(&json!({"click": {"world": [10, 10, 10]}})).unwrap();
+    h.frames(2);
+    assert_eq!(h.app.dialog.as_ref().map(|d| d.inputs[0].items.clone()), Some(picked), "no pick while computing");
+    finish.send(()).unwrap();
+    until_applied(&mut h);
+    assert!(h.app.dialog.is_none());
+    h.step(&json!({"expect": {"bodies": 1, "volume": 13000}})).unwrap();
+}
+
+/// Inputs that changed while OK computed (nothing should change them) are computed in turn, off
+/// the window thread too, then applied: run as OK finished, they would hold the window for the
+/// whole boolean.
+#[test]
+fn inputs_changed_while_ok_computes_are_computed_off_the_thread_too() {
+    let mut h = square_on_a_block();
+    h.app.apply_off_thread = Some(Duration::ZERO);
+    let features = h.app.session.doc.features.len();
+    // OK computing other commands than the dialog's, until the test lets it finish.
+    let (finish, done) = std::sync::mpsc::channel();
+    h.app.applying = Some(crate::dialogs::Applying { done, started: crate::now_ms(), cmds: Vec::new() });
+    finish.send(()).unwrap();
+    h.frames(1);
+    assert!(h.app.applying.is_some(), "the dialog's own commands are computing");
+    assert_eq!(h.app.session.doc.features.len(), features, "nothing applied on the window thread");
+    until_applied(&mut h);
+    assert!(h.app.dialog.is_none());
+    assert_eq!(h.app.session.doc.features.len(), features + 1);
+    h.step(&json!({"expect": {"bodies": 1, "volume": 13000}})).unwrap();
+}
diff --git a/crates/ui-egui/src/canvas.rs b/crates/ui-egui/src/canvas.rs
index 8bf25ed4..043534ac 100644
--- a/crates/ui-egui/src/canvas.rs
+++ b/crates/ui-egui/src/canvas.rs
@@ -199,6 +199,10 @@ fn edge_bisector(m: &Mesh, index: usize, p: Vec3) -> Option<Vec3> {
 
 /// Draw the manipulator and the value box (called by the viewport after the model is drawn).
 pub fn show(app: &mut SolveApp, ui: &mut egui::Ui, painter: &egui::Painter, proj: &Proj) {
+    // While OK computes, the inputs hold what it computes: no arrow to drag, no box to type in.
+    if app.applying.is_some() {
+        return;
+    }
     let Some(mut d) = app.dialog.take() else { return };
     if matches!(d.kind, Kind::Assembly(_)) {
         crate::dialogs_assembly::overlay(app, painter, proj, &mut d);
diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index d0fdcc1b..9346aceb 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -316,6 +316,61 @@ pub struct Dialog {
     pub then_start: Option<&'static str>,
 }
 
+/// OK computing off the window thread: the dialog's commands evaluated on a scratch copy of the
+/// session, which leaves the feature in the evaluation cache the session shares. Once that is
+/// in, OK runs them for real and finds the feature there; meanwhile the window stays live and
+/// the dialog says it is computing.
+pub struct Applying {
+    pub(crate) done: std::sync::mpsc::Receiver<()>,
+    /// When OK was pressed (ms).
+    pub(crate) started: f64,
+    /// The commands being computed.
+    pub(crate) cmds: Vec<(String, Value)>,
+}
+
+impl Applying {
+    /// The evaluation is in (or its worker ended).
+    fn finished(&self) -> bool {
+        !matches!(self.done.try_recv(), Err(std::sync::mpsc::TryRecvError::Empty))
+    }
+}
+
+/// How long the desktop app's OK waits for its evaluation before it lets the window go on and
+/// shows that it is computing ([`SolveApp::apply_off_thread`]): a feature quicker than this
+/// applies at once, as it always did.
+pub const APPLY_AT_ONCE: std::time::Duration = std::time::Duration::from_millis(150);
+
+/// Start OK's evaluation off the window thread, for a feature dialog. `None` when OK should
+/// just run: not a feature, not asked for ([`SolveApp::apply_off_thread`]), no threads (the
+/// web), or done within the wait.
+#[cfg(not(target_arch = "wasm32"))]
+fn begin_applying(app: &SolveApp, d: &Dialog) -> Option<Applying> {
+    let wait = app.apply_off_thread?;
+    if !d.previews() {
+        return None;
+    }
+    let cmds = apply_commands(app, d).ok()?;
+    let scratch = app.session.scratch();
+    let (tx, rx) = std::sync::mpsc::channel();
+    let computed = cmds.clone();
+    std::thread::Builder::new()
+        .name("apply".into())
+        .spawn(move || {
+            let _ = scratch.preview(&computed);
+            let _ = tx.send(());
+        })
+        .ok()?;
+    match rx.recv_timeout(wait) {
+        Err(std::sync::mpsc::RecvTimeoutError::Timeout) => Some(Applying { done: rx, started: crate::now_ms(), cmds }),
+        _ => None,
+    }
+}
+
+#[cfg(target_arch = "wasm32")]
+fn begin_applying(_app: &SolveApp, _d: &Dialog) -> Option<Applying> {
+    None
+}
+
 /// The commands a dialog's inputs make (or why they make none), its picks, and the design's
 /// revision (a parameter the commands name can change without changing them).
 #[derive(Clone, Debug, PartialEq)]
@@ -1881,6 +1936,10 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
         let footer_height = ui.spacing().interact_size.y.max(24.0) + 6.0 + ui.spacing().item_spacing.y;
         let fields_height = (vp.bottom() - anchor.y - 8.0 - vertical_frame - ui.min_rect().height() - footer_height).max(1.0);
         egui::ScrollArea::vertical().id_salt("feature_fields").min_scrolled_height(fields_height).max_height(fields_height).show(ui, |ui| {
+            // While OK computes, its inputs hold: what is applied is what was asked.
+            if app.applying.is_some() {
+                ui.disable();
+            }
             // Configuration tables own their column widths and scrolling; the two-column form
             // grid would squeeze their row names and action buttons into a label-sized cell.
             if let Kind::Motion(k @ crate::dialogs_motion::Mo::Configurations { .. }) = &mut d.kind {
@@ -2616,7 +2675,12 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
             // the automatic ids of OK and Cancel, and a click pressed in one frame and
             // released in the next would be lost (an animating preview toggles it often).
             let (sr, sresp) = ui.allocate_exact_size(vec2(14.0, 14.0), egui::Sense::hover());
-            if app.preview.busy {
+            if let Some(job) = &app.applying {
+                egui::Spinner::new().size(14.0).paint_at(ui, sr);
+                let seconds = ((crate::now_ms() - job.started) / 1000.0).floor().max(0.0);
+                ui.label(RichText::new(crate::i18n::format("dialog.computing", &[("seconds", &format!("{seconds:.0}"))])).color(t.text_dim));
+                ctx.request_repaint_after(std::time::Duration::from_millis(100));
+            } else if app.preview.busy {
                 egui::Spinner::new().size(14.0).paint_at(ui, sr);
                 sresp.on_hover_text(tr("Updating the preview"));
             }
@@ -2636,7 +2700,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                         .fill(if valid { t.accent } else { Color32::TRANSPARENT })
                         .wrap_mode(egui::TextWrapMode::Extend)
                         .min_size(vec2(ok_width, 24.0));
-                    let r = ui.add_enabled(valid, b);
+                    let r = ui.add_enabled(valid && app.applying.is_none(), b);
                     crate::scenario::publish_handle(&format!("dialog:{raw_label}"), r.rect.center());
                     if r.clicked() {
                         ok = true;
@@ -2661,7 +2725,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
     let enter_applies = !matches!(d.kind, Kind::Sketch | Kind::Params { .. } | Kind::Measure { .. } | Kind::ConfirmDelete { .. })
         && !matches!(&d.kind, Kind::Motion(k) if k.wide());
     let report_active = app.issue_report.draft.is_some();
-    if !report_active && enter_applies && (enter || canvas_enter || (enter_free && nothing_focused)) {
+    if !report_active && enter_applies && app.applying.is_none() && (enter || canvas_enter || (enter_free && nothing_focused)) {
         ok = true;
     }
     if !report_active && esc && !app.esc_handled {
@@ -2682,20 +2746,50 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
             Err(error) => d.refuse(app, error),
         }
     }
-    if ok {
+    if ok && app.applying.is_none() {
         // Edit Text applies its own way: its live changes and OK are one undo step.
         let run = match &mut d.kind {
-            Kind::Text(k) => crate::sketch_text::apply(app, k),
-            _ => run_dialog(app, &d),
+            Kind::Text(k) => Some(crate::sketch_text::apply(app, k)),
+            _ => match begin_applying(app, &d) {
+                // Computing: the dialog stays, saying so, until it is in.
+                Some(job) => {
+                    app.applying = Some(job);
+                    None
+                }
+                None => Some(run_dialog(app, &d)),
+            },
         };
         match run {
-            Ok(()) => {
+            Some(Ok(())) => {
                 cancel = true;
                 applied = true;
             }
-            Err(e) => d.refuse(app, e),
+            Some(Err(e)) => d.refuse(app, e),
+            None => {}
         }
     }
+    // OK's evaluation is in: run it, the feature taken from the cache. Inputs changed meanwhile
+    // (nothing should change them) are computed in turn, off the window thread too: run here,
+    // they would hold the window for the whole boolean.
+    if app.applying.as_ref().is_some_and(Applying::finished) && !cancel {
+        let computed = app.applying.take().map(|job| job.cmds);
+        let current = apply_commands(app, &d).ok();
+        let again = if current.is_some() && current != computed { begin_applying(app, &d) } else { None };
+        match again {
+            Some(job) => app.applying = Some(job),
+            None => match run_dialog(app, &d) {
+                Ok(()) => {
+                    cancel = true;
+                    applied = true;
+                }
+                Err(e) => d.refuse(app, e),
+            },
+        }
+    }
+    // Cancel while computing: the result goes (its worker runs on to its end).
+    if cancel && !applied {
+        app.applying = None;
+    }
     if !keep || cancel {
         crate::dialogs_motion::closed(app, &mut d, applied);
         if let Kind::Text(k) = &mut d.kind {
@@ -2724,6 +2818,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
 
 /// Close the dialog without applying it (Esc, Cancel). An edit puts the timeline marker back.
 pub fn cancel(app: &mut SolveApp) {
+    app.applying = None;
     if let Some(mut d) = app.dialog.take() {
         crate::dialogs_motion::closed(app, &mut d, false);
         if let Kind::Text(k) = &mut d.kind {
@@ -3032,8 +3127,6 @@ fn sels(d: &Dialog, i: usize) -> &[Sel] {
 
 pub(crate) fn run_dialog(app: &mut SolveApp, d: &Dialog) -> Result<(), String> {
     let commands = apply_commands(app, d)?;
-    // The live preview may be computing these very commands: OK takes its result.
-    crate::preview::settle(app, &commands);
     if let Some((_, marker)) = d.editing {
         // Record the actual pre-edit timeline state in the command's undo snapshot.
         roll_marker(app, marker)?;
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 95eeee85..5f6dee2b 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -78,6 +78,8 @@ mod status_tests {
         assert!(app.session.log.last().is_some_and(|l| l.starts_with("Redo: ") && !l.contains("edit.redo")), "{:?}", app.session.log.last());
     }
 }
+#[cfg(test)]
+mod apply_off_thread_tests;
 mod expression_complete;
 #[cfg(test)]
 mod extrude_edit_tests;
@@ -279,6 +281,11 @@ pub struct SolveApp {
     pub preferences: prefs::Prefs,
     pub prefs_window: prefs::PrefsWindow,
     pub preview: preview::PreviewState,
+    /// OK pressed on a feature that takes a while: its evaluation runs off the window thread.
+    pub applying: Option<dialogs::Applying>,
+    /// How long OK waits for a feature before computing it off the window thread, saying so;
+    /// `None`: it waits as long as it takes (headless use, tests). The desktop app sets it.
+    pub apply_off_thread: Option<std::time::Duration>,
     /// The last command started interactively (id, label), for Repeat.
     pub last_command: Option<(String, String)>,
     /// Esc was already acted on this frame (by the shortcuts), so later handlers skip it.
@@ -373,6 +380,8 @@ impl SolveApp {
             preferences: prefs::Prefs::default(),
             prefs_window: prefs::PrefsWindow::default(),
             preview: preview::PreviewState::default(),
+            applying: None,
+            apply_off_thread: None,
             last_command: None,
             esc_handled: false,
             status: None,
diff --git a/crates/ui-egui/src/preview.rs b/crates/ui-egui/src/preview.rs
index b59de0db..56117241 100644
--- a/crates/ui-egui/src/preview.rs
+++ b/crates/ui-egui/src/preview.rs
@@ -4,6 +4,10 @@
 //! tinted, removed material as a translucent ghost. The last good preview stays up while the
 //! next one computes or when an input makes the feature fail (the error shows in the dialog).
 //! Nothing here touches the document or the undo history.
+//!
+//! A feature that adds or removes a tool body (join, cut, intersect) shows only its tool. That
+//! is all its preview builds: its boolean, which can take seconds (a text joined onto a part),
+//! runs once, on OK, and an error it finds shows then, the dialog staying open.
 
 use std::hash::{Hash, Hasher};
 use std::sync::mpsc::{Receiver, TryRecvError};
@@ -60,18 +64,13 @@ pub struct Built {
 
 type Job = Result<Built, String>;
 
-/// What the preview worker sends: for a feature that adds or removes a tool body, the tool as
-/// soon as it is built (the boolean still running), then the finished preview.
-enum Msg {
-    Partial(Built),
-    Done(Job),
-}
-
 #[derive(Default)]
 pub struct PreviewState {
     /// Key of the commands whose result (or error) is shown.
     shown_key: Option<u64>,
-    running: Option<(u64, Receiver<Msg>)>,
+    running: Option<(u64, Receiver<Job>)>,
+    /// The running job builds a tool alone (for a join, cut or intersect).
+    running_tool: bool,
     /// Inputs of an in-flight joint preview, excluding only its driven values.
     running_motion_key: Option<u64>,
     /// Bodies the preview stands in for (hidden in the model scene).
@@ -102,6 +101,7 @@ pub struct PreviewState {
 impl PreviewState {
     fn clear(&mut self) {
         self.running = None;
+        self.running_tool = false;
         self.running_motion_key = None;
         self.shown_key = None;
         self.error = None;
@@ -157,28 +157,23 @@ impl PreviewState {
             self.sketches.clear();
             self.error = None;
         }
-        while let Some((k, rx)) = &self.running {
+        if let Some((k, rx)) = &self.running {
             let k = *k;
             let job = match rx.try_recv() {
-                // The tool, the boolean still running: shown, and the preview stays busy.
-                Ok(Msg::Partial(b)) => {
-                    if k == key {
-                        self.apply(k, Ok(b));
-                    }
-                    continue;
-                }
-                Ok(Msg::Done(job)) => Some(job),
+                Ok(job) => Some(job),
                 Err(TryRecvError::Empty) => None,
                 Err(TryRecvError::Disconnected) => Some(Err("the preview failed".into())),
             };
-            let Some(job) = job else { break };
-            {
+            if let Some(job) = job {
                 self.running = None;
-                let intermediate = motion_key.is_some() && self.running_motion_key == motion_key && matches!(&job, Ok(b) if b.error.is_none());
+                let good = matches!(&job, Ok(b) if b.error.is_none());
+                let intermediate = good && (self.running_tool || motion_key.is_some() && self.running_motion_key == motion_key);
                 self.running_motion_key = None;
-                // A held slider or animation may advance faster than the worker. Show
-                // completed poses from the same joint setup, then compute the latest.
-                // Errors and results from different setups still require an exact key.
+                self.running_tool = false;
+                // A held slider, a dragged arrow or an animation may advance faster than the
+                // worker. Show completed poses from the same joint setup, and tools built for
+                // inputs since changed, then compute the latest. Errors still require an exact
+                // key.
                 if k == key || intermediate {
                     self.apply(k, job);
                 }
@@ -233,13 +228,14 @@ pub fn update(app: &mut SolveApp, ctx: &egui::Context) {
     let colors = Colors::from_tokens(&Tokens::get());
     let key = job_key(&cmds, app.session.revision, colors);
     let motion_key = motion_key(&cmds, app.session.revision, colors);
+    let tool = as_new_body(&cmds).is_some();
     let pv = &mut app.preview;
     pv.poll(key, motion_key);
     if pv.running.is_none() && pv.shown_key != Some(key) {
         pv.started = crate::now_ms();
         pv.running_motion_key = motion_key;
         let scratch = app.session.scratch();
-        spawn(&mut app.preview, key, scratch, cmds, colors);
+        spawn(&mut app.preview, key, scratch, cmds, colors, tool);
     }
     app.preview.busy = app.preview.running.is_some();
     if app.preview.busy {
@@ -257,46 +253,26 @@ fn job_key(cmds: &[(String, Value)], revision: u64, colors: Colors) -> u64 {
     h.finish()
 }
 
-/// Before OK runs `cmds`: when the preview is still computing those very commands, wait for it
-/// rather than compute them a second time alongside it. What it evaluated stays in the model's
-/// cache, which the session shares with the preview's scratch copy, so OK then takes it from
-/// there (the feature, its result or its error) instead of evaluating it again.
-pub fn settle(app: &mut SolveApp, cmds: &[(String, Value)]) {
-    let key = job_key(cmds, app.session.revision, Colors::from_tokens(&Tokens::get()));
-    let Some((k, rx)) = app.preview.running.take_if(|(k, _)| *k == key) else { return };
-    loop {
-        match rx.recv() {
-            Ok(Msg::Partial(b)) => app.preview.apply(k, Ok(b)),
-            Ok(Msg::Done(job)) => {
-                app.preview.apply(k, job);
-                break;
-            }
-            Err(_) => break,
-        }
-    }
-    app.preview.running_motion_key = None;
-    app.preview.busy = false;
-}
-
+/// Start computing the preview of `cmds`: for a tool feature (`tool`), only its tool.
 #[cfg(not(target_arch = "wasm32"))]
-fn spawn(pv: &mut PreviewState, key: u64, scratch: Session, cmds: Vec<(String, Value)>, colors: Colors) {
+fn spawn(pv: &mut PreviewState, key: u64, scratch: Session, cmds: Vec<(String, Value)>, colors: Colors, tool: bool) {
     let (tx, rx) = std::sync::mpsc::channel();
     let spawned = std::thread::Builder::new().name("preview".into()).spawn(move || {
-        let job = compute_staged(&scratch, &cmds, colors, &mut |b| {
-            let _ = tx.send(Msg::Partial(b));
-        });
-        let _ = tx.send(Msg::Done(job));
+        let _ = tx.send(if tool { compute_tool(&scratch, &cmds, colors) } else { compute(&scratch, &cmds, colors) });
     });
     match spawned {
-        Ok(_) => pv.running = Some((key, rx)),
+        Ok(_) => {
+            pv.running = Some((key, rx));
+            pv.running_tool = tool;
+        }
         Err(e) => pv.apply(key, Err(e.to_string())),
     }
 }
 
 /// No threads on the web: compute right away.
 #[cfg(target_arch = "wasm32")]
-fn spawn(pv: &mut PreviewState, key: u64, scratch: Session, cmds: Vec<(String, Value)>, colors: Colors) {
-    pv.apply(key, compute(&scratch, &cmds, colors));
+fn spawn(pv: &mut PreviewState, key: u64, scratch: Session, cmds: Vec<(String, Value)>, colors: Colors, tool: bool) {
+    pv.apply(key, if tool { compute_tool(&scratch, &cmds, colors) } else { compute(&scratch, &cmds, colors) });
 }
 
 /// Evaluate the commands on the scratch session and build the preview scene.
@@ -305,13 +281,6 @@ fn spawn(pv: &mut PreviewState, key: u64, scratch: Session, cmds: Vec<(String, V
 /// show their result in place of the bodies they change. When the feature fails (a boolean the
 /// kernel can't do yet), the tool body is still shown, with the error.
 pub fn compute(s: &Session, cmds: &[(String, Value)], colors: Colors) -> Job {
-    compute_staged(s, cmds, colors, &mut |_| {})
-}
-
-/// [`compute`], handing a tool body's scene to `partial` as soon as it is built: the feature
-/// itself (the boolean, which the preview runs for its error and OK finds in the cache) can
-/// take far longer.
-fn compute_staged(s: &Session, cmds: &[(String, Value)], colors: Colors, partial: &mut dyn FnMut(Built)) -> Job {
     if cmds.iter().any(|(command, params)| {
         matches!(command.as_str(), "sketch.redefine" | "sketch.mirror" | "sketch.pattern.rectangular" | "sketch.pattern.circular" | "sketch.offset")
             || command == "construct.plane.along_path"
@@ -329,30 +298,16 @@ fn compute_staged(s: &Session, cmds: &[(String, Value)], colors: Colors, partial
     if let Some(job) = crate::dialogs_assembly::world_preview(s, cmds, colors) {
         return job;
     }
-    let guard = |f: &dyn Fn() -> Built| std::panic::catch_unwind(std::panic::AssertUnwindSafe(f)).map_err(|_| "the preview failed".to_string());
     match as_new_body(cmds) {
-        Some((alt, op)) => {
-            let tool = match s.preview(&alt) {
-                Ok(t) => t,
-                Err(e) => return Err(s.preview(cmds).err().map_or_else(|| e.to_string(), |r| r.to_string())),
-            };
-            let mut b = guard(&|| build_tool(&tool.before, &tool.after, colors, &op))?;
-            b.bounds = state_bounds(&tool.after);
-            partial(Built {
-                scene: b.scene.clone(),
-                sketches: Vec::new(),
-                replaced: b.replaced.clone(),
-                error: None,
-                bounds: b.bounds,
-                applied: None,
-                tapered: Vec::new(),
-                stopped: None,
-            });
+        Some(_) => {
+            let mut b = compute_tool(s, cmds, colors)?;
             b.error = s.preview(cmds).err().map(|e| e.to_string());
             Ok(b)
         }
         None => {
             let p = s.preview(cmds).map_err(|e| e.to_string())?;
+            let guard =
+                |f: &dyn Fn() -> Built| std::panic::catch_unwind(std::panic::AssertUnwindSafe(f)).map_err(|_| "the preview failed".to_string());
             let mut b = guard(&|| build(&p.before, &p.after, colors))?;
             b.bounds = state_bounds(&p.after);
             // The radius a previewed fillet used when it reduced the one asked for (a new
@@ -365,6 +320,21 @@ fn compute_staged(s: &Session, cmds: &[(String, Value)], colors: Colors, partial
     }
 }
 
+/// The tool body of a feature that adds or removes one, built on its own (as a new body): what
+/// its live preview shows, without the boolean. When even the tool fails, the feature's own
+/// error. Other features: their whole preview ([`compute`]).
+fn compute_tool(s: &Session, cmds: &[(String, Value)], colors: Colors) -> Job {
+    let Some((alt, op)) = as_new_body(cmds) else { return compute(s, cmds, colors) };
+    let tool = match s.preview(&alt) {
+        Ok(t) => t,
+        Err(e) => return Err(s.preview(cmds).err().map_or_else(|| e.to_string(), |r| r.to_string())),
+    };
+    let mut b = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| build_tool(&tool.before, &tool.after, colors, &op)))
+        .map_err(|_| "the preview failed".to_string())?;
+    b.bounds = state_bounds(&tool.after);
+    Ok(b)
+}
+
 fn state_bounds(st: &ModelState) -> Aabb3 {
     st.bodies.iter().fold(Aabb3::EMPTY, |b, x| b.union(&x.mesh().bounds()))
 }
@@ -711,7 +681,7 @@ mod tests {
         assert!(pv.active);
         assert_eq!(pv.replaced, ["Turn"]);
         assert_eq!(pv.key, 9, "the last good geometry stays visible");
-        tx.send(Msg::Done(Err("unknown_angle".into()))).unwrap();
+        tx.send(Err("unknown_angle".into())).unwrap();
         pv.poll(2, None);
         assert!(pv.error.is_none(), "a late obsolete result cannot disable Apply again");
         assert!(pv.running.is_none());
@@ -720,7 +690,7 @@ mod tests {
 
         let (tx, rx) = std::sync::mpsc::channel();
         pv.running = Some((2, rx));
-        tx.send(Msg::Done(Err("current invalid expression".into()))).unwrap();
+        tx.send(Err("current invalid expression".into())).unwrap();
         pv.poll(2, None);
         assert_eq!(pv.error.as_deref(), Some("current invalid expression"));
         assert_eq!(pv.shown_key, Some(2));
@@ -753,7 +723,7 @@ mod tests {
             pv.running_motion_key = Some(42);
             pv.poll(wanted, Some(42));
             assert!(pv.running.is_some());
-            tx.send(Msg::Done(Ok(pose()))).unwrap();
+            tx.send(Ok(pose())).unwrap();
             pv.poll(wanted, Some(42));
             assert!(pv.active, "motion should be visible before input stops");
             assert_eq!(pv.replaced, ["Lid"]);
@@ -764,7 +734,7 @@ mod tests {
         let (tx, rx) = std::sync::mpsc::channel();
         pv.running = Some((4, rx));
         pv.running_motion_key = Some(42);
-        tx.send(Msg::Done(Ok(pose()))).unwrap();
+        tx.send(Ok(pose())).unwrap();
         pv.poll(4, Some(42));
         assert_eq!(pv.shown_key, Some(4), "release catches up to the exact final pose");
         assert!(pv.running.is_none());
@@ -781,7 +751,7 @@ mod tests {
             let mut pv = PreviewState { running_motion_key: Some(42), ..PreviewState::default() };
             let (tx, rx) = std::sync::mpsc::channel();
             pv.running = Some((1, rx));
-            tx.send(Msg::Done(job)).unwrap();
+            tx.send(job).unwrap();
             pv.poll(2, context);
             assert!(!pv.active);
             assert!(pv.error.is_none(), "obsolete errors must not block Apply");
@@ -966,10 +936,17 @@ mod tests {
         assert!(s.model.state().bodies.is_empty());
     }
 
+    fn session_of(cmds: &[(&str, Value)]) -> Session {
+        let mut s = Session::default();
+        for (id, p) in cmds {
+            s.execute(id, p).unwrap();
+        }
+        s
+    }
+
     /// A block and a two-letter text to cut into it.
     fn text_on_block() -> (Session, Vec<(String, Value)>) {
-        let mut s = Session::default();
-        for (id, p) in [
+        let s = session_of(&[
             ("sketch.create", json!({"plane": "XY"})),
             ("sketch.rectangle.two_point", json!({"p0": [0, 0], "p1": [60, 20]})),
             ("sketch.finish", json!({})),
@@ -977,54 +954,46 @@ mod tests {
             ("sketch.create", json!({"plane": "XY"})),
             ("sketch.text", json!({"text": "AB", "at": [5, 5], "height": 10})),
             ("sketch.finish", json!({})),
-        ] {
-            s.execute(id, &p).unwrap();
-        }
+        ]);
         (s, vec![("solid.extrude".to_string(), json!({"distance": 2, "operation": "cut"}))])
     }
 
-    /// A cut's tool is handed over as soon as it is built, before the cut itself has run; the
-    /// finished preview is the same tool, with the cut's error if any.
+    /// A tool feature's live preview builds its tool without the boolean: a cut into nothing
+    /// shows its tool with no error (OK will report it), while the whole preview has the error.
     #[test]
-    fn a_cut_preview_shows_its_tool_before_the_boolean_runs() {
-        let (s, cmds) = text_on_block();
-        let mut partials = Vec::new();
-        let done = compute_staged(&s, &cmds, colors(), &mut |b| partials.push((b.scene.xray.len(), b.error.clone()))).unwrap();
-        assert_eq!(partials.len(), 1, "one early tool scene");
-        assert!(partials[0].0 > 0 && partials[0].1.is_none());
-        assert_eq!(done.scene.xray.len(), partials[0].0, "the cut tool, see-through");
-        assert!(done.error.is_none(), "{:?}", done.error);
-        // A feature with no tool body has nothing to show early.
-        let mut none = 0;
-        let fillet = vec![("solid.box".to_string(), json!({"length": 10, "width": 10, "height": 10}))];
-        compute_staged(&s, &fillet, colors(), &mut |_| none += 1).unwrap();
-        assert_eq!(none, 0);
+    fn a_tool_preview_builds_the_tool_without_the_boolean() {
+        let s = session_of(&[
+            ("sketch.create", json!({"plane": "XY"})),
+            ("sketch.rectangle.two_point", json!({"p0": [0, 0], "p1": [10, 10]})),
+            ("sketch.finish", json!({})),
+        ]);
+        let cmds = vec![("solid.extrude".to_string(), json!({"distance": 2, "operation": "cut"}))];
+        let tool = compute_tool(&s, &cmds, colors()).unwrap();
+        assert!(tool.error.is_none() && !tool.scene.xray.is_empty(), "{:?}", tool.error);
+        let whole = compute(&s, &cmds, colors()).unwrap();
+        assert!(whole.error.as_deref().is_some_and(|e| e.contains("no body to cut")), "{:?}", whole.error);
+        assert_eq!(whole.scene.xray.len(), tool.scene.xray.len());
     }
 
-    /// OK while the preview still computes the same commands waits for it instead of computing
-    /// them again beside it, and then takes the feature from the cache the preview filled.
+    /// While an arrow is dragged the inputs change faster than a tool is built: a tool built
+    /// for inputs since changed is shown anyway (then the latest computed), an error only for
+    /// the inputs it belongs to.
     #[test]
-    fn ok_takes_the_result_of_the_preview_still_computing() {
+    fn a_tool_for_inputs_since_changed_still_shows() {
         let (s, cmds) = text_on_block();
-        let mut app = SolveApp::new(s, crate::Services::default());
-        let colors = Colors::from_tokens(&Tokens::get());
-        // A preview of other commands is left alone.
-        let other = vec![("solid.extrude".to_string(), json!({"distance": 3, "operation": "cut"}))];
-        spawn(&mut app.preview, job_key(&other, app.session.revision, colors), app.session.scratch(), other.clone(), colors);
-        settle(&mut app, &cmds);
-        assert!(app.preview.running.is_some(), "not the commands OK runs");
-        let key = job_key(&cmds, app.session.revision, colors);
-        spawn(&mut app.preview, key, app.session.scratch(), cmds.clone(), colors);
-        settle(&mut app, &cmds);
-        assert!(app.preview.running.is_none() && !app.preview.busy);
-        assert_eq!(app.preview.shown_key, Some(key));
-        assert!(app.preview.active && app.preview.error.is_none(), "{:?}", app.preview.error);
-        let hits = |app: &SolveApp| app.session.model.cache.lock().map(|c| c.hits).unwrap_or(0);
-        let before = hits(&app);
-        let (id, params) = cmds[0].clone();
-        app.run(&id, params).unwrap();
-        let _ = app.session.world_state();
-        assert!(hits(&app) > before, "the cut came from the preview's evaluation");
-        assert_eq!(app.session.world_state().bodies.len(), 1);
+        let tool = compute_tool(&s, &cmds, colors()).unwrap();
+        let mut pv = PreviewState::default();
+        let (tx, rx) = std::sync::mpsc::channel();
+        pv.running = Some((1, rx));
+        pv.running_tool = true;
+        tx.send(Ok(tool)).unwrap();
+        pv.poll(2, None);
+        assert!(pv.running.is_none() && pv.active && pv.shown_key == Some(1), "shown, and key 2 still to compute");
+        let (tx, rx) = std::sync::mpsc::channel();
+        pv.running = Some((3, rx));
+        pv.running_tool = true;
+        tx.send(Err("too far".into())).unwrap();
+        pv.poll(4, None);
+        assert!(pv.error.is_none() && pv.shown_key == Some(1), "an old error is dropped");
     }
 }
diff --git a/crates/ui-egui/src/viewport.rs b/crates/ui-egui/src/viewport.rs
index d6557c7d..1db3dada 100644
--- a/crates/ui-egui/src/viewport.rs
+++ b/crates/ui-egui/src/viewport.rs
@@ -1444,6 +1444,10 @@ fn hit_candidate(app: &SolveApp, h: &Hit) -> Option<(Hit, Sel)> {
 
 /// A pick for the open dialog's active input (a click, or a Select Other row).
 pub(crate) fn dialog_pick(app: &mut SolveApp, hit: &Hit, sel: Sel) {
+    // While OK computes, the dialog's inputs hold.
+    if app.applying.is_some() {
+        return;
+    }
     let Some(mut d) = app.dialog.take() else { return };
     d.pick(&app.session, sel);
     if let crate::dialogs::Kind::PathPlane { sketch, .. } = &mut d.kind {
@@ -2730,6 +2734,10 @@ pub(crate) fn measurement_selection(s: &solidcraft_engine::Session, sel: &Sel) -
 /// active input takes (edges, faces or bodies), otherwise bodies.
 fn box_select(app: &mut SolveApp, proj: &Proj, bx: BoxSel, add: bool) {
     use crate::selection::{BODIES, EDGES, FACES, PROFILES};
+    // While OK computes, the dialog's inputs hold.
+    if app.applying.is_some() {
+        return;
+    }
     let st = app.session.world_state();
     let mut out: Vec<Sel> = Vec::new();
     let to2 = |pts: &[Vec3]| -> Vec<Pos2> { pts.iter().filter_map(|p| proj.to_screen(*p)).collect() };