feat(solid history): booleans keep the solids they joined editable

With SOLIDHIST on, UNION, SUBTRACT and INTERSECT no longer reduce the
result's history to a frozen body: each solid's history joins the
result's under a boolean step, in the order the result applied them.
The Properties palette lists a composite's solids under Operand; the one
chosen shows its own geometry rows and grips, placed where it sits in
the composite, and editing it replays the whole tree. A composite read
from a file keeps its saved body; the tree is replayed only on edits.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

locales/ar-SA/opencadstudio.ftl

@@ -2342,6 +2342,7 @@ common =
     .requires-two-positive-values = يتطلب قيمتين موجبتين (x,y) أو قيمة واحدة لكليهما.
     .snap-marker-color = لون علامة الالتقاط
     .command-line-text-color = لون نص سطر الأوامر
+    .operand = المعامل
     .linetypes-loaded = تم تحميل __ocs_fmt_0__ من أنواع الخطوط.
     .geometric-center-snap = المركز الهندسي
     .no-trim-option = بدون تشذيب

locales/bg-BG/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = Изискват се две положителни стойности (x,y) или една за двете.
     .snap-marker-color = Цвят на маркера за прихващане
     .command-line-text-color = Цвят на текста в командния ред
+    .operand = Операнд
     .linetypes-loaded = Заредени типове линии: __ocs_fmt_0__.
     .geometric-center-snap = Геометричен център
     .no-trim-option = Без подрязване

locales/cs-CZ/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = Vyžaduje dvě kladné hodnoty (x,y) nebo jednu pro obě.
     .snap-marker-color = Barva značky uchopení
     .command-line-text-color = Barva textu příkazového řádku
+    .operand = Operand
     .linetypes-loaded = Načteno typů čar: __ocs_fmt_0__.
     .geometric-center-snap = Geometrický střed
     .no-trim-option = Neoříznout

locales/de-DE/opencadstudio.ftl

@@ -2330,6 +2330,7 @@ common =
     .requires-two-positive-values = Erfordert zwei positive Werte (x,y) oder einen für beide.
     .snap-marker-color = Farbe der Fangmarkierung
     .command-line-text-color = Textfarbe der Befehlszeile
+    .operand = Operand
     .linetypes-loaded = __ocs_fmt_0__ Linientyp(en) geladen.
     .geometric-center-snap = Geometrischer Mittelpunkt
     .no-trim-option = Nicht stutzen

locales/el-GR/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = Απαιτούνται δύο θετικές τιμές (x,y) ή μία και για τις δύο.
     .snap-marker-color = Χρώμα δείκτη έλξης
     .command-line-text-color = Χρώμα κειμένου γραμμής εντολών
+    .operand = Τελεστέος
     .linetypes-loaded = Φορτώθηκαν __ocs_fmt_0__ τύποι γραμμής.
     .geometric-center-snap = Γεωμετρικό κέντρο
     .no-trim-option = Χωρίς περικοπή

locales/en-US/opencadstudio.ftl

@@ -2330,6 +2330,7 @@ common =
     .requires-two-positive-values = Requires two positive values (x,y) or one for both.
     .snap-marker-color = Snap marker color
     .command-line-text-color = Command line text color
+    .operand = Operand
     .linetypes-loaded = __ocs_fmt_0__ linetype(s) loaded.
     .geometric-center-snap = Geometric Center
     .no-trim-option = No trim

locales/es-ES/opencadstudio.ftl

@@ -2330,6 +2330,7 @@ common =
     .requires-two-positive-values = Requiere dos valores positivos (x,y) o uno para ambos.
     .snap-marker-color = Color del marcador de referencia
     .command-line-text-color = Color del texto de la línea de comandos
+    .operand = Operando
     .linetypes-loaded = __ocs_fmt_0__ tipo(s) de línea cargado(s).
     .geometric-center-snap = Centro geométrico
     .no-trim-option = No recortar

locales/fi-FI/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = Vaatii kaksi positiivista arvoa (x,y) tai yhden molemmille.
     .snap-marker-color = Kohdistusmerkin väri
     .command-line-text-color = Komentorivin tekstin väri
+    .operand = Operandi
     .linetypes-loaded = Ladattu __ocs_fmt_0__ viivatyyppiä.
     .geometric-center-snap = Geometrinen keskipiste
     .no-trim-option = Ei trimmausta

locales/fr-FR/opencadstudio.ftl

@@ -2330,6 +2330,7 @@ common =
     .requires-two-positive-values = Nécessite deux valeurs positives (x,y) ou une seule pour les deux.
     .snap-marker-color = Couleur du marqueur d'accrochage
     .command-line-text-color = Couleur du texte de la ligne de commande
+    .operand = Opérande
     .linetypes-loaded = __ocs_fmt_0__ type(s) de ligne chargé(s).
     .geometric-center-snap = Centre géométrique
     .no-trim-option = Sans ajuster

locales/hi-IN/opencadstudio.ftl

@@ -2318,6 +2318,7 @@ common =
     .requires-two-positive-values = दो धनात्मक मान (x,y) या दोनों के लिए एक मान आवश्यक है।
     .snap-marker-color = स्नैप मार्कर का रंग
     .command-line-text-color = कमांड लाइन टेक्स्ट रंग
+    .operand = ऑपरेंड
     .linetypes-loaded = __ocs_fmt_0__ लाइनटाइप लोड हुए।
     .geometric-center-snap = ज्यामितीय केंद्र
     .no-trim-option = ट्रिम नहीं

locales/hu-HU/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = Két pozitív érték (x,y) vagy egy közös érték szükséges.
     .snap-marker-color = Illesztési jelölő színe
     .command-line-text-color = Parancssor szövegszíne
+    .operand = Operandus
     .linetypes-loaded = __ocs_fmt_0__ vonaltípus betöltve.
     .geometric-center-snap = Geometriai középpont
     .no-trim-option = Nincs vágás

locales/it-IT/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = Richiede due valori positivi (x,y) o uno per entrambi.
     .snap-marker-color = Colore del marcatore di snap
     .command-line-text-color = Colore del testo della riga di comando
+    .operand = Operando
     .linetypes-loaded = __ocs_fmt_0__ tipo/i di linea caricato/i.
     .geometric-center-snap = Centro geometrico
     .no-trim-option = Non tagliare

locales/ja-JP/opencadstudio.ftl

@@ -2320,6 +2320,7 @@ common =
     .requires-two-positive-values = 2つの正の値 (x,y)、または両方に共通の1つの値が必要です。
     .snap-marker-color = スナップ マーカーの色
     .command-line-text-color = コマンドラインの文字色
+    .operand = オペランド
     .linetypes-loaded = __ocs_fmt_0__ 個の線種をロードしました。
     .geometric-center-snap = 図心
     .no-trim-option = トリムしない

locales/ko-KR/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = 양수 값 두 개(x,y) 또는 둘 다에 쓸 값 하나가 필요합니다.
     .snap-marker-color = 스냅 마커 색상
     .command-line-text-color = 명령줄 텍스트 색상
+    .operand = 피연산자
     .linetypes-loaded = 선종류 __ocs_fmt_0__개를 로드했습니다.
     .geometric-center-snap = 기하학적 중심
     .no-trim-option = 자르지 않기

locales/nl-NL/opencadstudio.ftl

@@ -2330,6 +2330,7 @@ common =
     .requires-two-positive-values = Vereist twee positieve waarden (x,y) of één voor beide.
     .snap-marker-color = Kleur van snapmarkering
     .command-line-text-color = Tekstkleur opdrachtregel
+    .operand = Operand
     .linetypes-loaded = __ocs_fmt_0__ lijntype(n) geladen.
     .geometric-center-snap = Geometrisch middelpunt
     .no-trim-option = Niet trimmen

locales/pl-PL/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = Wymaga dwóch wartości dodatnich (x,y) lub jednej dla obu.
     .snap-marker-color = Kolor znacznika przyciągania
     .command-line-text-color = Kolor tekstu wiersza poleceń
+    .operand = Operand
     .linetypes-loaded = Wczytano rodzaje linii: __ocs_fmt_0__.
     .geometric-center-snap = Środek geometryczny
     .no-trim-option = Bez przycinania

locales/pt-BR/opencadstudio.ftl

@@ -2330,6 +2330,7 @@ common =
     .requires-two-positive-values = Requer dois valores positivos (x,y) ou um para ambos.
     .snap-marker-color = Cor do marcador de snap
     .command-line-text-color = Cor do texto da linha de comando
+    .operand = Operando
     .linetypes-loaded = __ocs_fmt_0__ tipo(s) de linha carregado(s).
     .geometric-center-snap = Centro geométrico
     .no-trim-option = Não aparar

locales/ru-RU/opencadstudio.ftl

@@ -2333,6 +2333,7 @@ common =
     .requires-two-positive-values = Требуются два положительных значения (x,y) или одно для обоих.
     .snap-marker-color = Цвет маркера привязки
     .command-line-text-color = Цвет текста командной строки
+    .operand = Операнд
     .linetypes-loaded = Загружено типов линий: __ocs_fmt_0__.
     .geometric-center-snap = Геометрический центр
     .no-trim-option = Без обрезки

locales/tr-TR/opencadstudio.ftl

@@ -2314,6 +2314,7 @@ common =
     .requires-two-positive-values = İki pozitif değer (x,y) ya da ikisi için tek değer gerekir.
     .snap-marker-color = Yakalama işaretçisi rengi
     .command-line-text-color = Komut satırı metin rengi
+    .operand = İşlenen
     .linetypes-loaded = __ocs_fmt_0__ çizgi tipi yüklendi.
     .geometric-center-snap = Geometrik merkez
     .no-trim-option = Budama yok

locales/zh-CN/opencadstudio.ftl

@@ -2317,6 +2317,7 @@ common =
     .requires-two-positive-values = 需要两个正值 (x,y)，或一个同时用于两者的值。
     .snap-marker-color = 捕捉标记颜色
     .command-line-text-color = 命令行文字颜色
+    .operand = 操作对象
     .linetypes-loaded = 已加载 __ocs_fmt_0__ 个线型。
     .geometric-center-snap = 几何中心
     .no-trim-option = 不修剪

locales/zh-TW/opencadstudio.ftl

@@ -2329,6 +2329,7 @@ common =
     .requires-two-positive-values = 需要兩個正值 (x,y)，或一個同時用於兩者的值。
     .snap-marker-color = 鎖點標記顏色
     .command-line-text-color = 命令列文字顏色
+    .operand = 運算元
     .linetypes-loaded = 已載入 __ocs_fmt_0__ 個線型。
     .geometric-center-snap = 幾何中心
     .no-trim-option = 不修剪

src/app/model_ops.rs

@@ -4,7 +4,7 @@ use codec::{
     entities::{
         AcisData, EntityCommon, Region, Solid3D, Surface, SurfaceData, SurfaceKind, Wire,
     },
-    objects::SolidHistoryOperation,
+    objects::{SolidHistoryBoolean, SolidHistoryOperation},
     EntityType, Handle,
 };
 use kernel::brep::{Body, EdgeKey, FaceKey};
@@ -1006,6 +1006,32 @@ impl super::OpenCADStudio {
     }
 
     /// Intersect compatible selected solids, Regions, and coplanar Surfaces.
+    /// Before a boolean replaces `retained` with `entity`: with SOLIDHIST on,
+    /// join the histories of the solids it combined (`steps`, in the order
+    /// the result applied them) into `retained`'s and hand that history to
+    /// `entity`; otherwise drop it. True when the history was kept.
+    fn join_boolean_history(
+        &mut self,
+        retained: Handle,
+        entity: &mut EntityType,
+        steps: &[(Handle, u8)],
+        bodies: &HashMap<Handle, Body>,
+    ) -> bool {
+        let scene = &mut self.tabs[self.active_tab].scene;
+        let joined = scene.document.header.record_solid_history
+            && scene.join_boolean_histories(retained, retained, steps, bodies);
+        if !joined {
+            scene.delete_solid_history(retained);
+            return false;
+        }
+        if let (EntityType::Solid3D(entity), Some(EntityType::Solid3D(current))) =
+            (entity, scene.document.get_entity(retained))
+        {
+            entity.history_handle = current.history_handle;
+        }
+        true
+    }
+
     pub(super) fn solid_intersect(&mut self) -> Task<Message> {
         let i = self.active_tab;
         let mut handles = self.selected_intersect_handles();
@@ -1139,11 +1165,17 @@ impl super::OpenCADStudio {
             match group.outcome {
                 PreparedIntersectOutcome::Replace {
                     retained,
-                    entity,
+                    mut entity,
                     body,
                     display,
                 } => {
-                    self.tabs[i].scene.delete_solid_history(retained);
+                    let steps = group
+                        .handles
+                        .iter()
+                        .filter(|handle| **handle != retained)
+                        .map(|handle| (*handle, SolidHistoryBoolean::INTERSECT))
+                        .collect::<Vec<_>>();
+                    let joined = self.join_boolean_history(retained, &mut entity, &steps, &bodies);
                     if !self.tabs[i].scene.update_entity(entity) {
                         self.command_line.push_error(
                             crate::t!("INTERSECT: the retained object could not be updated.")
@@ -1152,6 +1184,7 @@ impl super::OpenCADStudio {
                         return Task::none();
                     }
                     if record_history
+                        && !joined
                         && matches!(
                             self.tabs[i].scene.document.get_entity(retained),
                             Some(EntityType::Solid3D(_))
@@ -1328,11 +1361,37 @@ impl super::OpenCADStudio {
                     let history = solid_history::brep_op(&group.body);
                     let mut entity = Solid3D::new();
                     entity.common = group.common;
-                    self.add_solid_model_preserving_style(
+                    let handle = self.add_solid_model_preserving_style(
                         EntityType::Solid3D(entity),
                         group.body,
                         history,
-                    )
+                    );
+                    // The new solid's history becomes the joined histories
+                    // of the solids it unites.
+                    let record = self.tabs[i].scene.document.header.record_solid_history;
+                    if !handle.is_null() && record {
+                        let bodies = group
+                            .handles
+                            .iter()
+                            .filter_map(|operand| {
+                                let body = self.tabs[i].scene.solid_models.get(operand)?;
+                                Some((*operand, body.clone()))
+                            })
+                            .collect::<HashMap<_, _>>();
+                        let steps = group
+                            .handles
+                            .iter()
+                            .skip(1)
+                            .map(|operand| (*operand, SolidHistoryBoolean::UNION))
+                            .collect::<Vec<_>>();
+                        let scene = &mut self.tabs[i].scene;
+                        if !scene.join_boolean_histories(handle, group.handles[0], &steps, &bodies) {
+                            if let Some(body) = scene.solid_models.get(&handle).cloned() {
+                                scene.create_solid_history(handle, solid_history::brep_op(&body));
+                            }
+                        }
+                    }
+                    handle
                 }
                 UnionEntityKind::Region => {
                     let mut entity = Region::new();
@@ -1588,8 +1647,20 @@ impl super::OpenCADStudio {
         let mut retained = Vec::new();
         let mut consumed = Vec::new();
         for group in prepared {
-            if let Some((handle, entity, body, display)) = group.result {
-                self.tabs[i].scene.delete_solid_history(handle);
+            if let Some((handle, mut entity, body, display)) = group.result {
+                let steps = group
+                    .bases
+                    .iter()
+                    .skip(1)
+                    .map(|base| (*base, SolidHistoryBoolean::UNION))
+                    .chain(
+                        group
+                            .cutters
+                            .iter()
+                            .map(|cutter| (*cutter, SolidHistoryBoolean::SUBTRACT)),
+                    )
+                    .collect::<Vec<_>>();
+                let joined = self.join_boolean_history(handle, &mut entity, &steps, &operand_bodies);
                 if !self.tabs[i].scene.update_entity(entity) {
                     self.command_line.push_error(
                         crate::t!("SUBTRACT: the retained base object could not be updated.").as_ref(),
@@ -1597,6 +1668,7 @@ impl super::OpenCADStudio {
                     return Task::none();
                 }
                 if record_history
+                    && !joined
                     && matches!(
                         self.tabs[i].scene.document.get_entity(handle),
                         Some(EntityType::Solid3D(_))

src/app/update/command.rs

@@ -2732,6 +2732,19 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                 self.tabs[i].properties.edit_choice_open = false;
                 return Task::none();
             }
+            if field == crate::scene::model::solid_history::PROP_OPERAND {
+                // Which solid of a composite to edit: palette state only.
+                for handle in &handles {
+                    crate::scene::model::solid_history::select_operand(
+                        &self.tabs[i].scene.document,
+                        *handle,
+                        &value,
+                    );
+                }
+                self.tabs[i].properties.edit_choice_open = false;
+                self.refresh_properties();
+                return Task::none();
+            }
             if crate::scene::model::solid_history::is_history_choice(field) {
                 self.apply_property_op(i, "CHPROP", &handles, |app, handle| {
                     if app.tabs[i].scene.is_layer_locked(handle) {

src/entities/object_data.rs

@@ -149,7 +149,7 @@ fn entity_history_handle(entity: &EntityType) -> Option<Handle> {
     .filter(|handle| handle.is_valid())
 }
 
-fn history_operation_name(operation: &SolidHistoryOperation) -> &'static str {
+pub(crate) fn history_operation_name(operation: &SolidHistoryOperation) -> &'static str {
     match operation {
         SolidHistoryOperation::Unknown => "Unknown",
         SolidHistoryOperation::Box(_) => "Box",

src/locale_catalog.rs

@@ -4895,6 +4895,7 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "Requires two positive values (x,y) or one for both." => Some(("common", "requires-two-positive-values")),
         "Snap marker color" => Some(("common", "snap-marker-color")),
         "Command line text color" => Some(("common", "command-line-text-color")),
+        "Operand" => Some(("common", "operand")),
         "{added} linetype(s) loaded." => Some(("common", "linetypes-loaded")),
         "Geometric Center" => Some(("common", "geometric-center-snap")),
         "No trim" => Some(("common", "no-trim-option")),

src/scene/entity.rs

@@ -756,11 +756,12 @@ impl Scene {
             .iter()
             .filter(|handle| !self.solid_models.contains_key(handle))
             .filter_map(|&handle| {
-                let from_history = self
-                    .document
-                    .solid_history_operations(handle)
-                    .and_then(|operations| kernel::acis::rebuild_history(&operations).ok());
-                let body = from_history.or_else(|| match self.document.get_entity(handle) {
+                let tree = self.document.solid_history_tree(handle);
+                let rebuilt = || {
+                    tree.as_ref()
+                        .and_then(|tree| kernel::acis::rebuild_history_tree(tree).ok())
+                };
+                let stored = || match self.document.get_entity(handle) {
                     Some(EntityType::Solid3D(solid)) => {
                         crate::scene::convert::solid3d_tess::kernel_body(solid)
                     }
@@ -771,7 +772,14 @@ impl Scene {
                         crate::scene::convert::solid3d_tess::kernel_surface_body(surface)
                     }
                     _ => None,
-                })?;
+                };
+                // A composite's saved body is what its booleans produced;
+                // replaying them is for edits, not for reading the file.
+                let body = if tree.as_ref().is_some_and(|tree| tree.has_boolean()) {
+                    stored().or_else(rebuilt)
+                } else {
+                    rebuilt().or_else(stored)
+                }?;
                 Some((handle, body))
             })
             .collect();

src/scene/model/solid_history.rs

@@ -94,6 +94,112 @@ pub const PROP_LOFT_CLOSED: &str = "solid_history_loft_closed";
 pub const PROP_LOFT_PERIODIC: &str = "solid_history_loft_periodic";
 pub const PROP_HISTORY: &str = "solid_history_record";
 pub const PROP_SHOW_HISTORY: &str = "solid_history_show";
+/// Which of a composite's solids the palette and grips edit.
+pub const PROP_OPERAND: &str = "solid_history_operand";
+
+thread_local! {
+    /// The operand edited inside each composite solid, by history node id.
+    static EDIT_OPERANDS: std::cell::RefCell<rustc_hash::FxHashMap<codec::Handle, i32>> =
+        std::cell::RefCell::new(rustc_hash::FxHashMap::default());
+}
+
+/// The solids a composite's booleans joined, as (node id, palette label) in
+/// operand order. Empty for a solid with no boolean step.
+pub fn composite_operands(
+    document: &codec::CadDocument,
+    handle: codec::Handle,
+) -> Vec<(i32, String)> {
+    let Some(tree) = document
+        .solid_history_tree(handle)
+        .filter(|tree| tree.has_boolean())
+    else {
+        return Vec::new();
+    };
+    tree.leaves()
+        .into_iter()
+        .enumerate()
+        .filter_map(|(index, operation)| {
+            let name = crate::entities::object_data::history_operation_name(operation);
+            Some((operation.base()?.node_id(), format!("{}: {}", index + 1, t!(name))))
+        })
+        .collect()
+}
+
+/// Edit the operand the palette lists as `label` from now on.
+pub fn select_operand(document: &codec::CadDocument, handle: codec::Handle, label: &str) {
+    if let Some((id, _)) = composite_operands(document, handle)
+        .into_iter()
+        .find(|(_, candidate)| candidate == label)
+    {
+        EDIT_OPERANDS.with(|operands| operands.borrow_mut().insert(handle, id));
+    }
+}
+
+/// The composite's history and the node id of the operand being edited: the
+/// one last chosen, or the first.
+fn selected_operand(
+    document: &codec::CadDocument,
+    handle: codec::Handle,
+) -> Option<(codec::objects::SolidHistoryTree, i32)> {
+    let tree = document
+        .solid_history_tree(handle)
+        .filter(|tree| tree.has_boolean())?;
+    let ids = tree
+        .leaves()
+        .into_iter()
+        .filter_map(|operation| Some(operation.base()?.node_id()))
+        .collect::<Vec<_>>();
+    let chosen = EDIT_OPERANDS.with(|operands| operands.borrow().get(&handle).copied());
+    let id = chosen.filter(|id| ids.contains(id)).or(ids.first().copied())?;
+    Some((tree, id))
+}
+
+/// The placement the steps above node `id` add to it: the product of their
+/// transforms, outermost first.
+fn ancestors_frame(tree: &codec::objects::SolidHistoryTree, id: i32) -> Option<glam::DMat4> {
+    if tree.operation.base().is_some_and(|base| base.node_id() == id) {
+        return Some(glam::DMat4::IDENTITY);
+    }
+    let own = matrix(tree.operation.base()?.transform)?;
+    tree.operands
+        .iter()
+        .find_map(|operand| ancestors_frame(operand, id))
+        .map(|inner| own * inner)
+}
+
+/// The operand of a composite solid that the palette and grips edit, placed
+/// where it sits in the composite. None for a solid with no boolean step.
+pub fn edit_operand(
+    document: &codec::CadDocument,
+    handle: codec::Handle,
+) -> Option<SolidHistoryOperation> {
+    let (tree, id) = selected_operand(document, handle)?;
+    let frame = ancestors_frame(&tree, id)?;
+    let mut operation = tree.find(id)?.operation.clone();
+    let base = operation.base_mut()?;
+    base.transform = (frame * matrix(base.transform)?).to_cols_array();
+    Some(operation)
+}
+
+/// An operand from [`edit_operand`], edited where it sits, back in the frame
+/// its composite stores it in. `operation` itself when the solid has no
+/// boolean step.
+pub fn operand_to_history(
+    document: &codec::CadDocument,
+    handle: codec::Handle,
+    mut operation: SolidHistoryOperation,
+) -> Option<SolidHistoryOperation> {
+    let Some(tree) = document
+        .solid_history_tree(handle)
+        .filter(|tree| tree.has_boolean())
+    else {
+        return Some(operation);
+    };
+    let frame = ancestors_frame(&tree, operation.base()?.node_id())?;
+    let base = operation.base_mut()?;
+    base.transform = (frame.inverse() * matrix(base.transform)?).to_cols_array();
+    Some(operation)
+}
 pub const PROP_SURFACE_TYPE: &str = "surface_type";
 pub const PROP_SURFACE_WIREFRAME_TYPE: &str = "srf_wireframe_type";
 pub const PROP_SURFACE_U_ISOLINES: &str = "srf_u_isolines";
@@ -250,6 +356,9 @@ pub fn primitive_property_operation(
     document: &codec::CadDocument,
     handle: codec::Handle,
 ) -> Option<SolidHistoryOperation> {
+    if let Some(operand) = edit_operand(document, handle) {
+        return Some(operand);
+    }
     document
         .solid_history_operations(handle)
         .and_then(|operations| operations.into_iter().next())
@@ -1619,7 +1728,33 @@ pub fn primitive_properties(
         return brep_properties(document, handle);
     };
     corner_frame(&mut operation);
-    match &operation {
+    let mut sections = operation_properties(document, handle, &operation);
+    let operands = composite_operands(document, handle);
+    let selected = operation
+        .base()
+        .and_then(|base| operands.iter().find(|(id, _)| *id == base.node_id()));
+    if let (Some((_, selected)), Some(first)) = (selected, sections.first_mut()) {
+        first.props.insert(
+            0,
+            Property {
+                label: t!("Operand").into_owned(),
+                field: PROP_OPERAND,
+                value: PropValue::Choice {
+                    selected: selected.clone(),
+                    options: operands.iter().map(|(_, label)| label.clone()).collect(),
+                },
+            },
+        );
+    }
+    sections
+}
+
+fn operation_properties(
+    document: &codec::CadDocument,
+    handle: codec::Handle,
+    operation: &SolidHistoryOperation,
+) -> Vec<PropSection> {
+    match operation {
         SolidHistoryOperation::Box(value) => {
             rectangular_properties(document, handle, value, "Box")
         }
@@ -1706,7 +1841,7 @@ pub fn is_loft_geometry_choice(field: &str) -> bool {
 }
 
 pub fn is_specialized_property(field: &str) -> bool {
-    matches!(field, "solid_history_type" | PROP_BANK | PROP_SWEEP_LENGTH
+    matches!(field, "solid_history_type" | PROP_OPERAND | PROP_BANK | PROP_SWEEP_LENGTH
         | PROP_LOFT_TYPE | PROP_LOFT_SECTION_COUNT
         | PROP_EXTRUSION_DIRECTION_X | PROP_EXTRUSION_DIRECTION_Y | PROP_EXTRUSION_DIRECTION_Z
         | PROP_SURFACE_TYPE | PROP_SURFACE_WIREFRAME_TYPE
@@ -3372,18 +3507,14 @@ pub fn chamfer_distance_grips(
     handle: codec::Handle,
     value: &SolidHistoryChamfer,
 ) -> Vec<GripDef> {
-    let Some(operations) = document.solid_history_operations(handle) else {
-        return Vec::new();
-    };
-    let Some(chamfer_index) = operations
-        .iter()
-        .rposition(|operation| matches!(operation, SolidHistoryOperation::Chamfer(_)))
+    // The chamfer's edge ordinals name edges of the solid it was cut from.
+    let Some(source) = document
+        .solid_history_tree(handle)
+        .and_then(|tree| tree.find(value.base.node_id())?.operands.first().cloned())
+        .and_then(|operand| kernel::acis::rebuild_history_tree(&operand).ok())
     else {
         return Vec::new();
     };
-    let Ok(source) = kernel::acis::rebuild_history(&operations[..chamfer_index]) else {
-        return Vec::new();
-    };
     let edges = source.edge_keys().collect::<Vec<_>>();
     let faces = source.face_keys().collect::<Vec<_>>();
     let Some(edge) = value
@@ -3469,10 +3600,11 @@ pub fn primitive_grips(
     document: &codec::CadDocument,
     handle: codec::Handle,
 ) -> Vec<GripDef> {
-    let Some(operation) = document.solid_history_operation(handle) else {
+    let Some(mut operation) = edit_operand(document, handle)
+        .or_else(|| document.solid_history_operation(handle).cloned())
+    else {
         return Vec::new();
     };
-    let mut operation = operation.clone();
     corner_frame(&mut operation);
     let operation = &operation;
     if let SolidHistoryOperation::Fillet(value) = operation {

src/scene/modify.rs

@@ -913,6 +913,87 @@ impl Scene {
         true
     }
 
+    /// Make `target`'s history the boolean steps that built it: starting from
+    /// `first`'s history, each of `steps` joins a solid's history into the
+    /// result so far. When `first` is `target` its own history is kept; a
+    /// solid with no history starts from its body in `bodies`.
+    ///
+    /// False, with `target` left without a history, when one of the solids
+    /// cannot carry one.
+    pub fn join_boolean_histories(
+        &mut self,
+        target: Handle,
+        first: Handle,
+        steps: &[(Handle, u8)],
+        bodies: &std::collections::HashMap<Handle, kernel::brep::Body>,
+    ) -> bool {
+        let solids = std::iter::once(target)
+            .chain(std::iter::once(first))
+            .chain(steps.iter().map(|(handle, _)| *handle))
+            .collect::<Vec<_>>();
+        let solid = |handle: &Handle| {
+            matches!(self.document.get_entity(*handle), Some(EntityType::Solid3D(_)))
+        };
+        if !solids.iter().all(solid) {
+            return false;
+        }
+        for handle in solids.iter().skip(1) {
+            if self.document.solid_history_graph(*handle).is_some() {
+                continue;
+            }
+            let Some(body) = bodies.get(handle) else {
+                return false;
+            };
+            if !self.create_solid_history(
+                *handle,
+                crate::scene::model::solid_history::brep_op(body),
+            ) {
+                return false;
+            }
+        }
+        if first != target {
+            self.delete_solid_history(target);
+            if self.is_recording_undo() {
+                let before = self.document.get_entity_arc(target);
+                self.record_undo_before(target, before);
+            }
+            if !self.copy_solid_history(first, target) {
+                return false;
+            }
+        }
+        for &(tool, operation) in steps {
+            if !self.merge_solid_history_boolean(target, tool, operation) {
+                self.delete_solid_history(target);
+                return false;
+            }
+        }
+        true
+    }
+
+    fn merge_solid_history_boolean(&mut self, target: Handle, tool: Handle, operation: u8) -> bool {
+        let previous: Vec<Handle> = [target, tool]
+            .iter()
+            .filter_map(|handle| self.document.solid_history_graph(*handle))
+            .flat_map(|graph| graph.nodes.into_iter().chain(graph.evaluation_graph))
+            .collect();
+        self.record_solid_history_before(target);
+        self.record_solid_history_before(tool);
+        if self.is_recording_undo() {
+            // The tool loses its history link; undo must give it back.
+            let before = self.document.get_entity_arc(tool);
+            self.record_undo_before(tool, before);
+        }
+        let Some(graph) = self.document.merge_solid_history_boolean(target, tool, operation) else {
+            return false;
+        };
+        for node in graph.nodes.into_iter().chain(graph.evaluation_graph) {
+            if !previous.contains(&node) {
+                self.record_undo_object_before(node, None);
+            }
+        }
+        true
+    }
+
     pub(crate) fn sync_solid_reference_point(&mut self, handle: Handle) {
         let reference = self
             .document
@@ -971,26 +1052,9 @@ impl Scene {
                 kernel::acis::rebuild_body(operation).ok()
             }
             (EntityType::Solid3D(_), _) => {
-                let mut operations = self.document.solid_history_operations(handle)?;
-                let replacement_id = operation.base().map(|base| {
-                    if base.eval.node_id > 0 {
-                        base.eval.node_id
-                    } else {
-                        base.step_id
-                    }
-                })?;
-                let target = operations.iter_mut().find(|candidate| {
-                    candidate.base().is_some_and(|base| {
-                        let node_id = if base.eval.node_id > 0 {
-                            base.eval.node_id
-                        } else {
-                            base.step_id
-                        };
-                        node_id == replacement_id
-                    })
-                })?;
-                *target = operation.clone();
-                kernel::acis::rebuild_history(&operations).ok()
+                let mut tree = self.document.solid_history_tree(handle)?;
+                tree.find_mut(operation.base()?.node_id())?.operation = operation.clone();
+                kernel::acis::rebuild_history_tree(&tree).ok()
             }
             _ => None,
         }
@@ -1126,11 +1190,18 @@ impl Scene {
         if self.history_surface_data(handle, &operation).is_none() {
             return false;
         }
-        if self
+        // An operand inside a composite is a step below the active one.
+        let active = self
             .document
-            .update_solid_history(handle, operation)
-            .is_none()
-        {
+            .solid_history_operation(handle)
+            .and_then(|active| active.base())
+            .map(|base| base.node_id());
+        let updated = if operation.base().map(|base| base.node_id()) == active {
+            self.document.update_solid_history(handle, operation).is_some()
+        } else {
+            self.document.update_solid_history_step(handle, operation).is_some()
+        };
+        if !updated {
             return false;
         }
         let (isoline_counts, planar_isolines) = match self.document.get_entity(handle) {
@@ -1270,7 +1341,10 @@ impl Scene {
         grip_id: usize,
         apply: GripApply,
     ) -> bool {
-        let Some(mut operation) = self.document.solid_history_operation(handle).cloned() else {
+        let Some(mut operation) =
+            crate::scene::model::solid_history::edit_operand(&self.document, handle)
+                .or_else(|| self.document.solid_history_operation(handle).cloned())
+        else {
             return false;
         };
         if grip_id == crate::scene::model::solid_history::GRIP_FILLET_RADIUS {
@@ -1345,6 +1419,11 @@ impl Scene {
         {
             return false;
         }
+        let Some(operation) =
+            crate::scene::model::solid_history::operand_to_history(&self.document, handle, operation)
+        else {
+            return false;
+        };
         self.preview_solid_history(handle, operation)
     }
 
@@ -1367,6 +1446,11 @@ impl Scene {
         ) {
             return false;
         }
+        let Some(operation) =
+            crate::scene::model::solid_history::operand_to_history(&self.document, handle, operation)
+        else {
+            return false;
+        };
         self.rebuild_solid_history(handle, operation)
     }