From 987f0be21ebcf9fee070dd919a30d75c50fe3610 Mon Sep 17 00:00:00 2001
From: DuBearte <130190089+DuBearte@users.noreply.github.com>
Date: Thu, 15 Oct 2026 22:54:16 -0400
Subject: [PATCH] Sketch: infinite construction lines (#1359) (#1547)

Thanks @DuBearte.
---
 assets/icons/ui/source/icons.rs               |  19 ++-
 assets/locales/de.json                        |   2 +
 assets/locales/en.json                        |   2 +
 assets/locales/fr.json                        |   2 +
 assets/locales/ja.json                        |   2 +
 assets/locales/ko.json                        |   2 +
 assets/locales/ru.json                        |   2 +
 assets/locales/uk.json                        |   2 +
 assets/locales/zh.json                        |   2 +
 .../format/pending/infinite-sketch-lines.rs   |   5 +
 crates/engine/src/cmd/features.rs             |   3 +-
 crates/engine/src/cmd/sketch.rs               |   8 +-
 crates/engine/src/cmd/sketch_clipboard.rs     |   4 +-
 crates/engine/src/cmd/sketch_create.rs        |  28 ++++
 .../engine/src/cmd/sketch_infinite_tests.rs   | 137 ++++++++++++++++++
 crates/engine/src/cmd/sketch_modify.rs        |  42 +++++-
 crates/engine/src/cmd/sketch_support.rs       |  14 ++
 crates/sketch/src/link.rs                     |   1 +
 crates/sketch/src/model.rs                    |  10 +-
 crates/sketch/src/offset_topology.rs          |  12 +-
 crates/sketch/src/profiles.rs                 |   2 +-
 crates/ui-egui/src/context_menu.rs            |  14 ++
 .../src/infinite_line_display_tests.rs        |  61 ++++++++
 crates/ui-egui/src/sketch_display.rs          |  35 ++++-
 crates/ui-egui/src/sketch_palette.rs          |  45 ++++--
 crates/ui-egui/src/tools.rs                   |   4 +-
 crates/ui-egui/src/viewport.rs                |   3 +-
 27 files changed, 435 insertions(+), 28 deletions(-)
 create mode 100644 crates/doc/src/format/pending/infinite-sketch-lines.rs
 create mode 100644 crates/engine/src/cmd/sketch_infinite_tests.rs
 create mode 100644 crates/ui-egui/src/infinite_line_display_tests.rs

diff --git a/assets/icons/ui/source/icons.rs b/assets/icons/ui/source/icons.rs
index 3a354f68..8f4fb5de 100644
--- a/assets/icons/ui/source/icons.rs
+++ b/assets/icons/ui/source/icons.rs
@@ -490,6 +490,23 @@ pub(crate) fn paint_with(p: &Painter, r: Rect, name: &str, ink: Color32, fill: C
             }
             p.circle_filled(at(0.5), dot_r, a);
         }
+        "infinite" => {
+            // An infinite line (#1359): Construction's broken line, with an open arrowhead at
+            // each end saying it runs on. Accent only, like Construction and Centerline.
+            let w = line_type_stroke(r);
+            let at = |t: f32| pen.at(3.0 + 16.0 * t, 19.0 - 16.0 * t);
+            for (t0, t1) in [(0.0, 0.38), (0.62, 1.0)] {
+                p.add(Shape::line(vec![at(t0), at(t1)], Stroke::new(w, a)));
+            }
+            // Each head: two short strokes back from the tip, either side of the line.
+            let tip_len = 0.3;
+            for (tip, back) in [(0.0f32, tip_len), (1.0, 1.0 - tip_len)] {
+                let (t, b) = (at(tip), at(back));
+                let along = b - t;
+                let side = egui::vec2(-along.y, along.x) * 0.55;
+                p.add(Shape::line(vec![b + side, t, b - side], Stroke::new(w, a)));
+            }
+        }
         "c_hv" => {
             pen.line(&[(4.0, 18.0), (20.0, 18.0)], a);
             pen.line(&[(6.0, 4.0), (6.0, 14.0)], a);
@@ -1482,7 +1499,7 @@ mod tests {
     /// theme's construction orange) and must not change with `ink`.
     #[test]
     fn construction_and_centerline_icons_follow_the_callers_accent_only() {
-        for name in ["construction", "centerline"] {
+        for name in ["construction", "centerline", "infinite"] {
             let base = drawn(name, Color32::BLACK, Color32::WHITE, Color32::from_rgb(1, 2, 3));
             let other_accent = drawn(name, Color32::BLACK, Color32::WHITE, Color32::from_rgb(4, 5, 6));
             assert_ne!(base, other_accent, "{name} icon must change with the caller's accent colour");
diff --git a/assets/locales/de.json b/assets/locales/de.json
index 5ee1040f..bf66c50d 100644
--- a/assets/locales/de.json
+++ b/assets/locales/de.json
@@ -305,6 +305,7 @@
   "cmd.sketch_fit_curves_to_section": "Kurven an Netzquerschnitt anpassen",
   "cmd.sketch_glyphs": "Bedingungssymbole",
   "cmd.sketch_include_3d": "3D-Geometrie einbeziehen",
+  "cmd.sketch_infinite": "Unendliche Linie",
   "cmd.sketch_insert_dxf": "DXF einfügen",
   "cmd.sketch_insert_svg": "SVG einfügen",
   "cmd.sketch_inspect": "Skizze prüfen",
@@ -907,6 +908,7 @@
   "ui.home": "Start",
   "ui.homeView": "Startansicht",
   "ui.horizontal_vertical": "Horizontal/Vertikal",
+  "ui.infinite": "Unendlich",
   "ui.insert": "Einfügen",
   "ui.inspect": "Prüfen",
   "ui.intersect": "Schnitt",
diff --git a/assets/locales/en.json b/assets/locales/en.json
index 768d01a1..c4f790a1 100644
--- a/assets/locales/en.json
+++ b/assets/locales/en.json
@@ -507,6 +507,7 @@
   "cmd.sketch_fit_curves_to_section": "Fit Curves to Mesh Section",
   "cmd.sketch_glyphs": "Constraint Glyphs",
   "cmd.sketch_include_3d": "Include 3D Geometry",
+  "cmd.sketch_infinite": "Infinite Line",
   "cmd.sketch_insert_dxf": "Insert DXF",
   "cmd.sketch_insert_svg": "Insert SVG",
   "cmd.sketch_inspect": "Inspect Sketch",
@@ -1139,6 +1140,7 @@
   "ui.homeView": "Home View",
   "ui.horizontal_vertical": "Horizontal/Vertical",
   "ui.import_ellipsis": "Import…",
+  "ui.infinite": "Infinite",
   "ui.insert": "Insert",
   "ui.inspect": "Inspect",
   "ui.intersect": "Intersect",
diff --git a/assets/locales/fr.json b/assets/locales/fr.json
index f4884104..df64badb 100644
--- a/assets/locales/fr.json
+++ b/assets/locales/fr.json
@@ -487,6 +487,7 @@
   "cmd.sketch_fit_curves_to_section": "Ajuster des courbes à la section du maillage",
   "cmd.sketch_glyphs": "Symboles de contraintes",
   "cmd.sketch_include_3d": "Inclure une géométrie 3D",
+  "cmd.sketch_infinite": "Ligne infinie",
   "cmd.sketch_insert_dxf": "Insérer un DXF",
   "cmd.sketch_insert_svg": "Insérer un SVG",
   "cmd.sketch_inspect": "Inspecter l'esquisse",
@@ -979,6 +980,7 @@
   "ui.homeView": "Vue d'accueil",
   "ui.horizontal_vertical": "Horizontale/Verticale",
   "ui.import_ellipsis": "Importer…",
+  "ui.infinite": "Infinie",
   "ui.insert": "Insérer",
   "ui.inspect": "Inspecter",
   "ui.intersect": "Intersection",
diff --git a/assets/locales/ja.json b/assets/locales/ja.json
index 8afc3832..b1ef7be0 100644
--- a/assets/locales/ja.json
+++ b/assets/locales/ja.json
@@ -507,6 +507,7 @@
   "cmd.sketch_fit_curves_to_section": "メッシュ断面に曲線をフィット",
   "cmd.sketch_glyphs": "拘束記号",
   "cmd.sketch_include_3d": "3D ジオメトリを含める",
+  "cmd.sketch_infinite": "無限線",
   "cmd.sketch_insert_dxf": "DXF を挿入",
   "cmd.sketch_insert_svg": "SVG を挿入",
   "cmd.sketch_inspect": "スケッチを検査",
@@ -1139,6 +1140,7 @@
   "ui.homeView": "ホームビュー",
   "ui.horizontal_vertical": "水平/垂直",
   "ui.import_ellipsis": "インポート…",
+  "ui.infinite": "無限",
   "ui.insert": "挿入",
   "ui.inspect": "検査",
   "ui.intersect": "交差",
diff --git a/assets/locales/ko.json b/assets/locales/ko.json
index dc99a66a..611179b5 100644
--- a/assets/locales/ko.json
+++ b/assets/locales/ko.json
@@ -305,6 +305,7 @@
   "cmd.sketch_fit_curves_to_section": "메시 단면에 곡선 맞추기",
   "cmd.sketch_glyphs": "구속 기호",
   "cmd.sketch_include_3d": "3D 기하 포함",
+  "cmd.sketch_infinite": "무한선",
   "cmd.sketch_insert_dxf": "DXF 삽입",
   "cmd.sketch_insert_svg": "SVG 삽입",
   "cmd.sketch_inspect": "스케치 검사",
@@ -907,6 +908,7 @@
   "ui.home": "홈",
   "ui.homeView": "홈 보기",
   "ui.horizontal_vertical": "수평/수직",
+  "ui.infinite": "무한",
   "ui.insert": "삽입",
   "ui.inspect": "검사",
   "ui.intersect": "교차",
diff --git a/assets/locales/ru.json b/assets/locales/ru.json
index 0022d10a..e7d0e2b4 100644
--- a/assets/locales/ru.json
+++ b/assets/locales/ru.json
@@ -458,6 +458,7 @@
   "cmd.sketch_fit_curves_to_section": "Подобрать кривые по сечению меша",
   "cmd.sketch_glyphs": "Знаки ограничений",
   "cmd.sketch_include_3d": "Включить 3D-геометрию",
+  "cmd.sketch_infinite": "Бесконечная линия",
   "cmd.sketch_insert_dxf": "Вставить DXF",
   "cmd.sketch_insert_svg": "Вставить SVG",
   "cmd.sketch_inspect": "Инспекция эскиза",
@@ -931,6 +932,7 @@
   "ui.homeView": "Главный вид",
   "ui.horizontal_vertical": "Горизонталь/Вертикаль",
   "ui.import_ellipsis": "Импорт…",
+  "ui.infinite": "Бесконечная",
   "ui.insert": "Вставить",
   "ui.inspect": "Инспекция",
   "ui.intersect": "Пересечение",
diff --git a/assets/locales/uk.json b/assets/locales/uk.json
index cde0098b..15b0e1b4 100644
--- a/assets/locales/uk.json
+++ b/assets/locales/uk.json
@@ -455,6 +455,7 @@
   "cmd.sketch_fit_curves_to_section": "Підігнати криві до перерізу сітки",
   "cmd.sketch_glyphs": "Позначки обмежень",
   "cmd.sketch_include_3d": "Включити 3D-геометрію",
+  "cmd.sketch_infinite": "Нескінченна лінія",
   "cmd.sketch_insert_dxf": "Вставити DXF",
   "cmd.sketch_insert_svg": "Вставити SVG",
   "cmd.sketch_inspect": "Перевірити ескіз",
@@ -830,6 +831,7 @@
   "ui.homeView": "Головний вид",
   "ui.horizontal_vertical": "Горизонталь/вертикаль",
   "ui.import_ellipsis": "Імпорт…",
+  "ui.infinite": "Нескінченна",
   "ui.insert": "Вставити",
   "ui.inspect": "Перевірити",
   "ui.intersect": "Перетин",
diff --git a/assets/locales/zh.json b/assets/locales/zh.json
index e32c8632..ed6d0c8d 100644
--- a/assets/locales/zh.json
+++ b/assets/locales/zh.json
@@ -507,6 +507,7 @@
   "cmd.sketch_fit_curves_to_section": "将曲线拟合到网格截面",
   "cmd.sketch_glyphs": "约束符号",
   "cmd.sketch_include_3d": "包含三维几何",
+  "cmd.sketch_infinite": "无限长直线",
   "cmd.sketch_insert_dxf": "插入 DXF",
   "cmd.sketch_insert_svg": "插入 SVG",
   "cmd.sketch_inspect": "检查草图",
@@ -1139,6 +1140,7 @@
   "ui.homeView": "主视图",
   "ui.horizontal_vertical": "水平/垂直",
   "ui.import_ellipsis": "导入…",
+  "ui.infinite": "无限",
   "ui.insert": "插入",
   "ui.inspect": "检查",
   "ui.intersect": "相交",
diff --git a/crates/doc/src/format/pending/infinite-sketch-lines.rs b/crates/doc/src/format/pending/infinite-sketch-lines.rs
new file mode 100644
index 00000000..1893a5d3
--- /dev/null
+++ b/crates/doc/src/format/pending/infinite-sketch-lines.rs
@@ -0,0 +1,5 @@
+//! A sketch line can be `infinite` (#1359): construction geometry through its two points, drawn
+//! across the whole view, a boundary for trim and extend along its whole line. Saved only when
+//! set: older designs need no transformation, and a line without it is the line it always was;
+//! 0.13.x (which reads up to 24) asks for an update instead of drawing such a line between its
+//! two points.
diff --git a/crates/engine/src/cmd/features.rs b/crates/engine/src/cmd/features.rs
index 8c6944c1..c9055cb4 100644
--- a/crates/engine/src/cmd/features.rs
+++ b/crates/engine/src/cmd/features.rs
@@ -593,7 +593,8 @@ fn thin_param(s: &Session, p: &Value, sketch: u64, cmd: &str) -> Result<ThinWall
     let ss = st.sketch(sketch).ok_or_else(|| bad(cmd, "the sketch is not evaluated"))?;
     let mut curves = string_list(p, "curves");
     if curves.is_empty() {
-        curves = ss.sketch.curves.iter().filter(|c| !c.construction && !c.centerline && c.link.is_none()).map(|c| c.id.clone()).collect();
+        curves =
+            ss.sketch.curves.iter().filter(|c| !c.construction && !c.centerline && !c.infinite && c.link.is_none()).map(|c| c.id.clone()).collect();
     }
     if curves.is_empty() || curves.len() > 1000 {
         return Err(bad(cmd, "a thin extrude needs 1…1000 sketch curves (`curves`)"));
diff --git a/crates/engine/src/cmd/sketch.rs b/crates/engine/src/cmd/sketch.rs
index f1e5d9b3..6cc69a3d 100644
--- a/crates/engine/src/cmd/sketch.rs
+++ b/crates/engine/src/cmd/sketch.rs
@@ -2244,11 +2244,14 @@ fn construction(s: &mut Session, p: &Value) -> Result<Value> {
         let mut n = 0;
         for c in cs {
             if let Some(cu) = sk.curves.get_mut(c)
-                && (cu.construction != to || to && cu.centerline)
+                && (cu.construction != to || to && cu.centerline || !to && cu.infinite)
             {
                 cu.construction = to;
                 if to {
                     cu.centerline = false;
+                } else {
+                    // A normal line is a finite one.
+                    cu.infinite = false;
                 }
                 n += 1;
             }
@@ -2512,6 +2515,9 @@ mod drag_live_tests;
 #[path = "sketch_ids_tests.rs"]
 mod ids_tests;
 #[cfg(test)]
+#[path = "sketch_infinite_tests.rs"]
+mod infinite_tests;
+#[cfg(test)]
 #[path = "sketch_redundant_tests.rs"]
 mod redundant_tests;
 #[cfg(test)]
diff --git a/crates/engine/src/cmd/sketch_clipboard.rs b/crates/engine/src/cmd/sketch_clipboard.rs
index b8036f93..c9764455 100644
--- a/crates/engine/src/cmd/sketch_clipboard.rs
+++ b/crates/engine/src/cmd/sketch_clipboard.rs
@@ -36,6 +36,7 @@ pub struct ClipCurve {
     pub construction: bool,
     pub centerline: bool,
     pub reversed: bool,
+    pub infinite: bool,
 }
 
 /// A constraint or dimension of the clipboard, over clipboard points and curves. A dimension
@@ -133,7 +134,7 @@ fn copy(s: &mut Session, p: &Value) -> Result<Value> {
         kind.map_points(&|q| used.iter().find(|(o, _)| *o == q).map_or(q, |(_, n)| *n));
         cmap.insert(c, clip.curves.len());
         // Projected geometry comes in as ordinary geometry.
-        clip.curves.push(ClipCurve { kind, construction: cu.construction, centerline: cu.centerline, reversed: cu.reversed });
+        clip.curves.push(ClipCurve { kind, construction: cu.construction, centerline: cu.centerline, reversed: cu.reversed, infinite: cu.infinite });
     }
     for &q in &ps {
         let known = pmap.contains_key(&q);
@@ -276,6 +277,7 @@ fn paste(s: &mut Session, p: &Value) -> Result<Value> {
                 cu.construction = c.construction;
                 cu.centerline = c.centerline;
                 cu.reversed = c.reversed;
+                cu.infinite = c.infinite;
             }
             made.push(i);
         }
diff --git a/crates/engine/src/cmd/sketch_create.rs b/crates/engine/src/cmd/sketch_create.rs
index e6d428d4..9cf50fbb 100644
--- a/crates/engine/src/cmd/sketch_create.rs
+++ b/crates/engine/src/cmd/sketch_create.rs
@@ -94,6 +94,9 @@ pub static COMMANDS: &[CommandSpec] = &[
         .icon("centerline")
         .enabled(in_sketch)
         .params("curves: [line ids], value?: bool (default toggles): centerline line type (not in profiles; an axis)"),
+    CommandSpec::new("sketch.infinite", "Infinite Line", infinite_line)
+        .enabled(in_sketch)
+        .params("curves: [line ids], value?: bool (default toggles): infinite line type (#1359): construction geometry drawn across the whole view through the line's two points; off, a normal line"),
     CommandSpec::new("sketch.text", "Text", text).at("SKETCH", "CREATE").icon("text").enabled(in_sketch).params(concat!(
         "text: string (\\n breaks lines), at: baseline start [x,y], height?: capital height mm (default 5), angle?: deg; at's x and y, height and angle each take a number or an expression over the design's parameters as a string (\"plate_width / 2\", \"label_size\"): the text keeps the expression, follows the parameters, and returns it in `exprs`; an expression naming no parameter is the number it comes to, a bare number in one is in the design's units, \
          font?: an installed font's family or full name, or a .ttf/.otf file path (~ and %VAR% expand; default: the built-in Liberation Sans), ",
@@ -436,6 +439,7 @@ fn centerline(s: &mut Session, p: &Value) -> Result<Value> {
             cu.centerline = want.unwrap_or(!cu.centerline);
             if cu.centerline {
                 cu.construction = false;
+                cu.infinite = false;
             }
         }
         Ok(ids.len())
@@ -443,6 +447,30 @@ fn centerline(s: &mut Session, p: &Value) -> Result<Value> {
     Ok(json!({"changed": n, "sketch": info}))
 }
 
+fn infinite_line(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "sketch.infinite";
+    let ids = string_list(p, "curves");
+    if ids.is_empty() {
+        return Err(bad(cmd, "`curves` must list lines"));
+    }
+    let want = crate::params::bool_(p, "value");
+    let (n, info) = edit(s, p, cmd, false, |sk, _| {
+        for id in &ids {
+            let c = sk.curve_index(id).ok_or_else(|| bad(cmd, format!("unknown curve `{id}`")))?;
+            let cu = sk.curves.get_mut(c).ok_or_else(|| bad(cmd, "curve"))?;
+            if !matches!(cu.kind, CurveKind::Line { .. }) {
+                return Err(bad(cmd, format!("`{id}` is not a line")));
+            }
+            // Infinite lines are construction geometry (never in a profile); off, a normal line.
+            cu.infinite = want.unwrap_or(!cu.infinite);
+            cu.construction = cu.infinite;
+            cu.centerline = false;
+        }
+        Ok(ids.len())
+    })?;
+    Ok(json!({"changed": n, "sketch": info}))
+}
+
 fn ellipse(s: &mut Session, p: &Value) -> Result<Value> {
     let cmd = "sketch.ellipse";
     let (c, m) = (req_vec2(cmd, p, "center")?, req_vec2(cmd, p, "major")?);
diff --git a/crates/engine/src/cmd/sketch_infinite_tests.rs b/crates/engine/src/cmd/sketch_infinite_tests.rs
new file mode 100644
index 00000000..11fff7ad
--- /dev/null
+++ b/crates/engine/src/cmd/sketch_infinite_tests.rs
@@ -0,0 +1,137 @@
+//! Infinite lines (#1359): a line type through two points, construction geometry that reaches
+//! across the whole view, a boundary along its whole line, never trimmed, filleted or in a
+//! profile.
+
+use serde_json::{Value, json};
+use solidcraft_geom::Vec2;
+use solidcraft_sketch::{CurveKind, Sketch};
+
+use crate::Session;
+
+fn run(s: &mut Session, id: &str, p: Value) -> Value {
+    match s.execute(id, &p) {
+        Ok(v) => v,
+        Err(e) => panic!("{id} {p}: {e}"),
+    }
+}
+
+fn sketch(s: &Session) -> Sketch {
+    s.doc.sketch(s.active_sketch.unwrap()).unwrap().clone()
+}
+
+fn curve<'a>(sk: &'a Sketch, id: &str) -> &'a solidcraft_sketch::Curve {
+    sk.curves.get(sk.curve_index(id).unwrap()).unwrap()
+}
+
+fn new_sketch() -> Session {
+    let mut s = Session::default();
+    run(&mut s, "sketch.create", json!({"plane": "XY"}));
+    s
+}
+
+fn profiles(s: &Session) -> usize {
+    s.model.state().sketch(s.active_sketch.unwrap()).unwrap().profiles.len()
+}
+
+#[test]
+fn the_line_type_switches_on_and_off_and_stays_out_of_profiles() {
+    let mut s = new_sketch();
+    run(&mut s, "sketch.rectangle.two_point", json!({"p0": [0, 0], "p1": [40, 20]}));
+    run(&mut s, "sketch.line", json!({"points": [[20, -10], [20, 30]], "ids": ["axis"]}));
+    assert_eq!(profiles(&s), 2, "a normal line cuts the rectangle in two");
+    run(&mut s, "sketch.infinite", json!({"curves": ["axis"]}));
+    let sk = sketch(&s);
+    assert!(curve(&sk, "axis").infinite && curve(&sk, "axis").construction && !curve(&sk, "axis").centerline);
+    assert_eq!(profiles(&s), 1, "an infinite line is construction: no profile");
+    // Back to a normal line, by toggling or by Normal/Construction.
+    run(&mut s, "sketch.infinite", json!({"curves": ["axis"]}));
+    let sk = sketch(&s);
+    assert!(!curve(&sk, "axis").infinite && !curve(&sk, "axis").construction);
+    run(&mut s, "sketch.infinite", json!({"curves": ["axis"], "value": true}));
+    run(&mut s, "sketch.construction", json!({"curves": ["axis"], "value": false}));
+    assert!(!curve(&sketch(&s), "axis").infinite, "a normal line is a finite one");
+    // A centerline is not an infinite line.
+    run(&mut s, "sketch.infinite", json!({"curves": ["axis"], "value": true}));
+    run(&mut s, "sketch.centerline", json!({"curves": ["axis"], "value": true}));
+    let sk = sketch(&s);
+    assert!(curve(&sk, "axis").centerline && !curve(&sk, "axis").infinite);
+    // Only lines.
+    run(&mut s, "sketch.circle.center", json!({"center": [60, 0], "radius": 5, "id": "c"}));
+    assert!(s.execute("sketch.infinite", &json!({"curves": ["c"]})).is_err());
+}
+
+#[test]
+fn it_is_saved_and_a_line_without_it_reads_finite() {
+    let mut s = new_sketch();
+    run(&mut s, "sketch.line", json!({"points": [[0, 0], [10, 5]], "ids": ["a"]}));
+    run(&mut s, "sketch.line", json!({"points": [[0, 10], [10, 15]], "ids": ["b"]}));
+    run(&mut s, "sketch.infinite", json!({"curves": ["a"]}));
+    let text = s.doc.to_json();
+    let back = solidcraft_doc::Document::from_json(&text).unwrap();
+    let sk = back.sketch(s.active_sketch.unwrap()).unwrap();
+    assert!(curve(sk, "a").infinite);
+    assert!(!curve(sk, "b").infinite);
+    assert_eq!(text.matches("\"infinite\"").count(), 1, "saved only where it is set");
+}
+
+#[test]
+fn trim_and_extend_use_the_whole_line_but_never_trim_it() {
+    // An infinite vertical line at x = 25 whose two points lie far above the other line.
+    let mut s = new_sketch();
+    run(&mut s, "sketch.line", json!({"points": [[25, 50], [25, 60]], "ids": ["inf"]}));
+    run(&mut s, "sketch.infinite", json!({"curves": ["inf"]}));
+    run(&mut s, "sketch.line", json!({"points": [[0, 0], [40, 0]], "ids": ["l"]}));
+    // Trimming the line's right part stops at the infinite line's crossing, x = 25.
+    run(&mut s, "sketch.trim", json!({"curve": "l", "at": [35, 0]}));
+    let sk = sketch(&s);
+    let CurveKind::Line { a, b } = curve(&sk, "l").kind else { panic!("a line") };
+    let xs = [sk.point(a).unwrap().x, sk.point(b).unwrap().x];
+    assert!(xs.iter().any(|x| (x - 25.0).abs() < 1e-6) && xs.iter().all(|x| *x <= 25.0 + 1e-6), "{xs:?}");
+    // The infinite line itself is not trimmed.
+    let e = s.execute("sketch.trim", &json!({"curve": "inf", "at": [25, 55]})).unwrap_err().to_string();
+    assert!(e.contains("infinite"), "{e}");
+    // A short line extends to it, beyond its two points.
+    run(&mut s, "sketch.line", json!({"points": [[0, 10], [10, 10]], "ids": ["short"]}));
+    run(&mut s, "sketch.extend", json!({"curve": "short", "at": [10, 10]}));
+    let sk = sketch(&s);
+    let CurveKind::Line { b, .. } = curve(&sk, "short").kind else { panic!("a line") };
+    assert!(sk.point(b).unwrap().dist(Vec2::new(25.0, 10.0)) < 1e-6, "{:?}", sk.point(b));
+}
+
+#[test]
+fn fillets_and_chamfers_refuse_it_and_mirrors_copy_it() {
+    let mut s = new_sketch();
+    run(&mut s, "sketch.line", json!({"points": [[0, 0], [20, 0], [20, 20]], "ids": ["a", "b"]}));
+    run(&mut s, "sketch.infinite", json!({"curves": ["a"]}));
+    let before = s.doc.to_json();
+    for (cmd, p) in [
+        ("sketch.fillet", json!({"point": "a.end", "radius": 2})),
+        ("sketch.fillet", json!({"a": "a", "b": "b", "radius": 2})),
+        ("sketch.chamfer.equal_distance", json!({"point": "a.end", "distance": 2})),
+    ] {
+        let e = s.execute(cmd, &p).unwrap_err().to_string();
+        assert!(e.contains("infinite"), "{cmd}: {e}");
+    }
+    assert_eq!(s.doc.to_json(), before);
+    run(&mut s, "sketch.line", json!({"points": [[-10, -5], [-10, 5]], "ids": ["m"]}));
+    let r = run(&mut s, "sketch.mirror", json!({"entities": ["a"], "line": "m"}));
+    let made = r["curves"].as_array().unwrap().iter().filter_map(Value::as_str).map(str::to_string).collect::<Vec<_>>();
+    let sk = sketch(&s);
+    assert!(made.iter().any(|id| curve(&sk, id).infinite), "{r}");
+}
+
+#[test]
+fn an_offset_of_it_is_infinite_too() {
+    let mut s = new_sketch();
+    run(&mut s, "sketch.line", json!({"points": [[0, 0], [10, 0]], "ids": ["inf"]}));
+    run(&mut s, "sketch.infinite", json!({"curves": ["inf"]}));
+    let r = run(&mut s, "sketch.offset", json!({"curves": ["inf"], "distance": 5, "side": [5, 5]}));
+    let made = r["curves"].as_array().unwrap().iter().filter_map(Value::as_str).map(str::to_string).collect::<Vec<_>>();
+    let sk = sketch(&s);
+    assert!(!made.is_empty() && made.iter().all(|id| curve(&sk, id).infinite), "{r}");
+    // It stays infinite when the offset follows a new distance.
+    let param = r["param"].as_str().unwrap().to_string();
+    run(&mut s, "parameters.change", json!({"name": param, "expression": "8 mm"}));
+    let sk = sketch(&s);
+    assert!(made.iter().all(|id| curve(&sk, id).infinite));
+}
diff --git a/crates/engine/src/cmd/sketch_modify.rs b/crates/engine/src/cmd/sketch_modify.rs
index 65e6325d..6c6249ee 100644
--- a/crates/engine/src/cmd/sketch_modify.rs
+++ b/crates/engine/src/cmd/sketch_modify.rs
@@ -147,8 +147,10 @@ fn cut_points(sk: &Sketch, ci: usize) -> Vec<(f64, Vec2, usize)> {
         {
             pts.extend([other.point(0.0), other.point(sweep)]);
         }
+        // An infinite line (#1359) cuts wherever its line crosses, beyond its two points too.
+        let other_infinite = sk.curves.get(oi).is_some_and(|c| c.infinite);
         for q in pts {
-            if on_bounded(&other, q).is_some()
+            if (other_infinite || on_bounded(&other, q).is_some())
                 && let Some(t) = on_bounded(&sh, q)
                 && !out.iter().any(|(_, x, _)| x.dist(q) < TOL * 10.0)
             {
@@ -197,7 +199,7 @@ fn split_at(sk: &mut Sketch, ci: usize, ts: &[f64]) -> Result<(Vec<usize>, Vec<u
     let mut pieces = vec![ci];
     let mut push = |sk: &mut Sketch, kind: CurveKind, prefix: &str| -> Result<()> {
         let id = sk.fresh(prefix)?;
-        sk.curves.push(Curve { id, kind, construction, reversed: false, link: None, centerline: false, fixed: false });
+        sk.curves.push(Curve { id, kind, construction, reversed: false, link: None, centerline: false, fixed: false, infinite: false });
         pieces.push(sk.curves.len() - 1);
         Ok(())
     };
@@ -286,6 +288,9 @@ fn trim(s: &mut Session, p: &Value) -> Result<Value> {
     let (out, info) = edit(s, p, cmd, false, |sk, doc| {
         let ci = curve_arg(sk, p, cmd)?;
         let id = sk.curves.get(ci).map(|c| c.id.clone()).unwrap_or_default();
+        if sk.curves.get(ci).is_some_and(|c| c.infinite) {
+            return Err(bad(cmd, "an infinite line has no ends to trim back to: make it a normal line first"));
+        }
         if sk.curves.get(ci).is_some_and(|c| c.kind.is_freeform()) {
             let r = super::sketch_freeform::trim_or_break(sk, ci, at, true, cmd)?;
             doc.prune_model_params();
@@ -418,7 +423,11 @@ fn extend(s: &mut Session, p: &Value) -> Result<Value> {
             if oi == ci {
                 continue;
             }
-            let pieces = sk.exact_segs(oi).unwrap_or_else(|| sk.segs(oi));
+            let mut pieces = sk.exact_segs(oi).unwrap_or_else(|| sk.segs(oi));
+            // An infinite line (#1359) is a boundary along its whole line.
+            if sk.curves.get(oi).is_some_and(|c| c.infinite) {
+                pieces = pieces.iter().map(|s| support::endless(s, sh.point(sh.param(at)))).collect();
+            }
             segment_count += pieces.len();
             if segment_count > 16_384 {
                 return Err(bad(cmd, "too many exact curve pieces to extend safely"));
@@ -528,6 +537,16 @@ fn weld_corner(sk: &mut Sketch, p: &Value) {
     }
 }
 
+/// Fillets and chamfers refuse a corner on an infinite line (#1359): it has no end there.
+fn refuse_infinite_corner(sk: &Sketch, p: &Value, cmd: &str) -> Result<()> {
+    let infinite = |c: Option<usize>| c.and_then(|c| sk.curves.get(c)).is_some_and(|c| c.infinite);
+    let at_corner = match str_(p, "point").and_then(|r| sk.resolve_point(r)) {
+        Some(q) => sk.curves.iter().any(|c| c.infinite && c.kind.uses(q)),
+        None => ["a", "b"].iter().any(|k| infinite(str_(p, k).and_then(|i| sk.curve_index(i)))),
+    };
+    if at_corner { Err(bad(cmd, "an infinite line has no corner to round or cut: make it a normal line first")) } else { Ok(()) }
+}
+
 /// A corner of two lines: (line a, line b, corner point, far end of a, far end of b).
 fn corner(sk: &Sketch, p: &Value, cmd: &str) -> Result<(usize, usize, usize, usize, usize)> {
     let ends = |l: usize| match sk.curves.get(l).map(|c| &c.kind) {
@@ -745,6 +764,7 @@ fn kept_json(kept: Vec<CornerKept>) -> (Vec<String>, Vec<String>) {
 /// Round one corner (`point` or `a`, `b`) with an arc of radius `r` tangent to both curves:
 /// the arc, the tangent constraints and what became of the corner point.
 fn fillet_corner(sk: &mut Sketch, p: &Value, r: f64, cmd: &str) -> Result<(usize, Vec<String>, CornerKept)> {
+    refuse_infinite_corner(sk, p, cmd)?;
     weld_corner(sk, p);
     let (la, lb, c, fa, fb) = match corner(sk, p, cmd) {
         Ok(x) => x,
@@ -854,6 +874,7 @@ fn round_corner(sk: &mut Sketch, p: &Value, r: f64, cmd: &str) -> Result<Option<
 
 fn chamfer(s: &mut Session, p: &Value, cmd: &str, dists: impl FnOnce(f64) -> Result<(f64, f64)>) -> Result<Value> {
     let (out, info) = edit(s, p, cmd, false, |sk, _| {
+        refuse_infinite_corner(sk, p, cmd)?;
         weld_corner(sk, p);
         let (la, lb, c, fa, fb) = corner(sk, p, cmd)?;
         let (pc, pa, pb) = (sk.point(c).unwrap_or_default(), sk.point(fa).unwrap_or_default(), sk.point(fb).unwrap_or_default());
@@ -1343,6 +1364,18 @@ fn offset(s: &mut Session, p: &Value) -> Result<Value> {
                 .map(|o| o.sources.iter().map(|(id, _)| id.clone()).collect::<Vec<_>>())
                 .or_else(|| sk.freeform_offsets.iter().find(|o| o.output == *output).map(|o| vec![o.source.clone()]))
                 .ok_or_else(|| bad(cmd, "offset association is missing"))?;
+            // An offset of infinite lines is infinite lines too (#1359).
+            let infinite = sources.iter().all(|id| sk.curve_index(id).and_then(|c| sk.curves.get(c)).is_some_and(|c| c.infinite));
+            if infinite {
+                for id in group.iter().flat_map(|side| side.curves.iter()) {
+                    if let Some(cu) = sk.curve_index(id).and_then(|c| sk.curves.get_mut(c))
+                        && matches!(cu.kind, CurveKind::Line { .. })
+                    {
+                        cu.infinite = true;
+                        cu.construction = true;
+                    }
+                }
+            }
             for (index, side) in group.iter_mut().enumerate() {
                 let name = if index == 1 { param2.as_ref().unwrap_or(&param) } else { &param };
                 let output = side.curves.first().ok_or_else(|| bad(cmd, "offset produced no curves"))?;
@@ -1631,6 +1664,8 @@ fn copy_through(
             }
         };
         let id = sk.fresh(prefix)?;
+        // A mirrored or patterned infinite line is an infinite line too (#1359).
+        let infinite = cu.infinite && matches!(kind, CurveKind::Line { .. });
         sk.curves.push(Curve {
             id,
             kind,
@@ -1639,6 +1674,7 @@ fn copy_through(
             link: None,
             centerline: false,
             fixed: false,
+            infinite,
         });
         made.push(sk.curves.len() - 1);
     }
diff --git a/crates/engine/src/cmd/sketch_support.rs b/crates/engine/src/cmd/sketch_support.rs
index 1a3208ef..1f3b1543 100644
--- a/crates/engine/src/cmd/sketch_support.rs
+++ b/crates/engine/src/cmd/sketch_support.rs
@@ -154,6 +154,20 @@ pub(super) fn hits(support: &Shape, seg: &Seg2, budget: &mut usize) -> Result<Ve
     Ok(out)
 }
 
+/// How far an infinite line (#1359) reaches each way from `near`, as a boundary: well past any
+/// sketch geometry.
+const ENDLESS: f64 = 1e7;
+
+/// An infinite line's piece `seg` as a boundary: the line through it, as a segment reaching
+/// [`ENDLESS`] each way from the point of the line nearest `near`. Other pieces are returned as
+/// they are.
+pub(super) fn endless(seg: &Seg2, near: Vec2) -> Seg2 {
+    let Seg2::Line { a, b } = *seg else { return *seg };
+    let Some(dir) = (b - a).normalized() else { return *seg };
+    let mid = a + dir * (near - a).dot(dir);
+    Seg2::Line { a: mid - dir * ENDLESS, b: mid + dir * ENDLESS }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/crates/sketch/src/link.rs b/crates/sketch/src/link.rs
index 4f57b9ba..8a8c274a 100644
--- a/crates/sketch/src/link.rs
+++ b/crates/sketch/src/link.rs
@@ -655,6 +655,7 @@ impl Sketch {
                 link: Some(id.to_string()),
                 centerline: false,
                 fixed: false,
+                infinite: false,
             });
         }
         for li in &lay.lone {
diff --git a/crates/sketch/src/model.rs b/crates/sketch/src/model.rs
index 2a483cc2..042d968b 100644
--- a/crates/sketch/src/model.rs
+++ b/crates/sketch/src/model.rs
@@ -169,6 +169,11 @@ pub struct Curve {
     /// Fixed (locked): its points and radius are constants for the solver.
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub fixed: bool,
+    /// Infinite line type (#1359), for lines only: construction geometry drawn across the whole
+    /// view, through its two points, which fix where it lies and which way it runs. Never part of
+    /// a profile (it is construction too).
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub infinite: bool,
 }
 
 /// Geometric constraints and dimensions. Indices refer to `Sketch::points` (`p`, `q`) or
@@ -735,6 +740,7 @@ impl Sketch {
             link: None,
             centerline: false,
             fixed: false,
+            infinite: false,
         });
         Ok(self.curves.len() - 1)
     }
@@ -772,6 +778,7 @@ impl Sketch {
             link: None,
             centerline: false,
             fixed: false,
+            infinite: false,
         });
         Ok(self.curves.len() - 1)
     }
@@ -806,6 +813,7 @@ impl Sketch {
             link: None,
             centerline: false,
             fixed: false,
+            infinite: false,
         });
         Ok(self.curves.len() - 1)
     }
@@ -1376,7 +1384,7 @@ impl Sketch {
             }
         };
         let id = self.curve_id(id, prefix)?;
-        self.curves.push(Curve { id, kind, construction: false, reversed: false, link: None, centerline: false, fixed: false });
+        self.curves.push(Curve { id, kind, construction: false, reversed: false, link: None, centerline: false, fixed: false, infinite: false });
         Ok(self.curves.len() - 1)
     }
 
diff --git a/crates/sketch/src/offset_topology.rs b/crates/sketch/src/offset_topology.rs
index 0427d121..2d04e071 100644
--- a/crates/sketch/src/offset_topology.rs
+++ b/crates/sketch/src/offset_topology.rs
@@ -27,13 +27,21 @@ pub struct OffsetRoleFlags {
     pub centerline: bool,
     pub reversed: bool,
     pub fixed: bool,
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub infinite: bool,
 }
 impl OffsetRoleFlags {
     fn is_default(&self) -> bool {
         *self == Self::default()
     }
     fn of(curve: &Curve) -> Self {
-        Self { construction: curve.construction, centerline: curve.centerline, reversed: curve.reversed, fixed: curve.fixed }
+        Self {
+            construction: curve.construction,
+            centerline: curve.centerline,
+            reversed: curve.reversed,
+            fixed: curve.fixed,
+            infinite: curve.infinite,
+        }
     }
 }
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -614,6 +622,7 @@ impl Sketch {
                     link: None,
                     centerline: false,
                     fixed: false,
+                    infinite: false,
                 });
                 outputs.push(output);
             }
@@ -767,6 +776,7 @@ impl Sketch {
                         link: None,
                         centerline: role.flags.centerline,
                         fixed: role.flags.fixed,
+                        infinite: role.flags.infinite,
                     });
                 }
                 outputs.push(role.output.clone());
diff --git a/crates/sketch/src/profiles.rs b/crates/sketch/src/profiles.rs
index 1a0be1fb..2b6d4a96 100644
--- a/crates/sketch/src/profiles.rs
+++ b/crates/sketch/src/profiles.rs
@@ -558,7 +558,7 @@ fn find_profiles_bounded(sk: &Sketch, max_edges: usize) -> Vec<Profile> {
     let mut whole_graph_complete = true;
     let mut expected = std::collections::HashMap::<usize, Vec<usize>>::new();
     for (ci, c) in sk.curves.iter().enumerate() {
-        if c.construction || c.centerline {
+        if c.construction || c.centerline || c.infinite {
             continue;
         }
         if matches!(c.kind, CurveKind::Line { .. } | CurveKind::Arc { .. } | CurveKind::Circle { .. }) {
diff --git a/crates/ui-egui/src/context_menu.rs b/crates/ui-egui/src/context_menu.rs
index cad1081e..cc1d18e4 100644
--- a/crates/ui-egui/src/context_menu.rs
+++ b/crates/ui-egui/src/context_menu.rs
@@ -781,6 +781,20 @@ fn sketch_entity_items(app: &SolveApp) -> Vec<Item> {
     if !curves.is_empty() {
         v.push(cmd(app, "sketch.construction", "Normal / Construction").with(json!({ "curves": curves })));
     }
+    // Infinite lines (#1359): the selected lines switch together.
+    let lines: Vec<String> = curves
+        .iter()
+        .filter(|id| {
+            ss.sketch
+                .curve_index(id)
+                .and_then(|i| ss.sketch.curves.get(i))
+                .is_some_and(|c| matches!(c.kind, solidcraft_engine::sketch::CurveKind::Line { .. }))
+        })
+        .cloned()
+        .collect();
+    if !lines.is_empty() {
+        v.push(cmd(app, "sketch.infinite", "Infinite Line").with(json!({ "curves": lines })));
+    }
     if curves.len() == 1 && ss.sketch.freeform_offsets.iter().any(|offset| offset.output == curves[0]) {
         v.push(cmd(app, "sketch.offset.break", "Break Offset Association").with(json!({"curve":curves[0]})));
     }
diff --git a/crates/ui-egui/src/infinite_line_display_tests.rs b/crates/ui-egui/src/infinite_line_display_tests.rs
new file mode 100644
index 00000000..bbb3bcc5
--- /dev/null
+++ b/crates/ui-egui/src/infinite_line_display_tests.rs
@@ -0,0 +1,61 @@
+//! An infinite line (#1359) is drawn across the whole view and picked along its whole line,
+//! and the Sketch Palette's Infinite mode draws one.
+
+use serde_json::json;
+
+use crate::scenario::Harness;
+use crate::viewport::{Hit, pick, projection};
+
+/// An XY sketch with a short line from (0, 0) to (2, 0) made infinite, the view zoomed out.
+fn short_infinite_line() -> Harness {
+    let mut h = Harness::new();
+    h.app.home.open = false;
+    h.app.run("sketch.create", json!({"plane": "XY"})).unwrap();
+    h.app.run("sketch.line", json!({"points": [[0, 0], [2, 0]], "ids": ["inf"]})).unwrap();
+    h.app.run("sketch.infinite", json!({"curves": ["inf"]})).unwrap();
+    h.app.cam.distance = 400.0;
+    h.settle();
+    h
+}
+
+#[test]
+fn it_is_drawn_from_edge_to_edge_of_the_view() {
+    let h = short_infinite_line();
+    let rect = h.app.viewport.rect.unwrap();
+    let proj = projection(&h.app, rect);
+    let st = h.app.session.world_state();
+    let ss = st.sketch(h.app.session.active_sketch.unwrap()).unwrap();
+    let i = ss.sketch.curve_index("inf").unwrap();
+    let parts = super::curve_parts(&ss.sketch, i, &ss.plane, &proj, 1.0);
+    let xs: Vec<f32> = parts.iter().flatten().filter_map(|p| proj.to_screen(*p)).map(|p| p.x).collect();
+    let (lo, hi) = xs.iter().fold((f32::MAX, f32::MIN), |(lo, hi), x| (lo.min(*x), hi.max(*x)));
+    // Its two points are a few pixels apart; drawn, it spans the view.
+    assert!(lo <= rect.left() + 2.0 && hi >= rect.right() - 2.0, "drawn from {lo} to {hi} in {rect:?}");
+}
+
+#[test]
+fn it_is_picked_far_beyond_its_two_points() {
+    let h = short_infinite_line();
+    let rect = h.app.viewport.rect.unwrap();
+    let proj = projection(&h.app, rect);
+    let ss_plane = h.app.session.world_state().sketch(h.app.session.active_sketch.unwrap()).unwrap().plane;
+    let far = proj.to_screen(ss_plane.to_world(solidcraft_engine::geom::Vec2::new(150.0, 0.0))).unwrap();
+    assert!(rect.contains(far));
+    let hits = pick(&h.app, &proj, far);
+    assert!(hits.iter().any(|h| matches!(h, Hit::SketchCurve { id, .. } if id == "inf")), "{hits:?}");
+}
+
+#[test]
+fn infinite_mode_draws_infinite_lines() {
+    let mut h = Harness::new();
+    h.app.home.open = false;
+    h.app.run("sketch.create", json!({"plane": "XY"})).unwrap();
+    h.settle();
+    crate::sketch_palette::set_mode(&mut h.app, 3);
+    let r = h.app.run("sketch.line", json!({"points": [[0, 0], [10, 5]]})).unwrap();
+    crate::sketch_palette::apply_mode(&mut h.app, "sketch.line", &r);
+    let id = r["curves"][0].as_str().unwrap().to_string();
+    let sk = h.app.session.doc.sketch(h.app.session.active_sketch.unwrap()).unwrap();
+    let c = sk.curves.get(sk.curve_index(&id).unwrap()).unwrap();
+    assert!(c.infinite && c.construction, "{c:?}");
+}
diff --git a/crates/ui-egui/src/sketch_display.rs b/crates/ui-egui/src/sketch_display.rs
index fbb20c19..a1178b60 100644
--- a/crates/ui-egui/src/sketch_display.rs
+++ b/crates/ui-egui/src/sketch_display.rs
@@ -26,8 +26,35 @@ pub fn pixel_view(proj: &Proj, ppp: f32) -> PixelCurveView {
     PixelCurveView { vp: proj.vp, width_px: f64::from(proj.rect.width() * ppp), height_px: f64::from(proj.rect.height() * ppp) }
 }
 
+/// The pieces a curve is drawn and picked from. An infinite line (#1359) reaches across the whole
+/// view: through the point of its line nearest the view's centre, far past the view each way
+/// (the sampler clips it to the view). Only the display changes: the line keeps its two points.
+fn display_segs(sketch: &Sketch, index: usize, plane: &Plane, proj: &Proj) -> Vec<solidcraft_engine::geom::Seg2> {
+    use solidcraft_engine::geom::Seg2;
+    let segs = sketch.exact_segs(index).unwrap_or_else(|| sketch.segs(index));
+    if !sketch.curves.get(index).is_some_and(|c| c.infinite) {
+        return segs;
+    }
+    let target = proj.cam.target - plane.origin;
+    let centre = Vec2::new(target.dot(plane.x), target.dot(plane.y));
+    let reach = 4.0 * (proj.cam.distance.abs() + proj.cam.half_height().abs()) + 1.0;
+    segs.into_iter()
+        .map(|s| match s {
+            Seg2::Line { a, b } => match (b - a).normalized() {
+                Some(dir) => {
+                    let mid = a + dir * (centre - a).dot(dir);
+                    let reach = reach + mid.dist(centre);
+                    Seg2::Line { a: mid - dir * reach, b: mid + dir * reach }
+                }
+                None => s,
+            },
+            other => other,
+        })
+        .collect()
+}
+
 pub fn curve_with_budget(sketch: &Sketch, index: usize, plane: &Plane, proj: &Proj, ppp: f32, budget: &mut SamplingBudget) -> SampledPath {
-    let segments = sketch.exact_segs(index).unwrap_or_else(|| sketch.segs(index));
+    let segments = display_segs(sketch, index, plane, proj);
     let mut local = SamplingBudget::new(budget.remaining_vertices.min(4096), budget.remaining_nodes.min(65_536));
     let initial = local.clone();
     let mut out = SampledPath { parts: Vec::new(), quality: SamplingQuality::Complete, vertex_count: 0 };
@@ -47,7 +74,7 @@ pub fn curve_with_budget(sketch: &Sketch, index: usize, plane: &Plane, proj: &Pr
 pub fn curve_unclipped(sketch: &Sketch, index: usize, plane: &Plane, proj: &Proj, ppp: f32) -> SampledPath {
     let mut budget = SamplingBudget::new(4096, 65_536);
     let mut out = SampledPath { parts: Vec::new(), quality: SamplingQuality::Complete, vertex_count: 0 };
-    for segment in sketch.exact_segs(index).unwrap_or_else(|| sketch.segs(index)) {
+    for segment in display_segs(sketch, index, plane, proj) {
         let sampled = sample_segment_unclipped(&segment, plane, &pixel_view(proj, ppp), &mut budget);
         if sampled.quality != SamplingQuality::Complete {
             out.quality = sampled.quality;
@@ -244,3 +271,7 @@ pub fn curve_sample(app: &SolveApp, sketch: u64, index: usize, plane: &Plane, pr
         )),
     }
 }
+
+#[cfg(test)]
+#[path = "infinite_line_display_tests.rs"]
+mod infinite_line_display_tests;
diff --git a/crates/ui-egui/src/sketch_palette.rs b/crates/ui-egui/src/sketch_palette.rs
index ac376ec0..aa81e55a 100644
--- a/crates/ui-egui/src/sketch_palette.rs
+++ b/crates/ui-egui/src/sketch_palette.rs
@@ -83,7 +83,7 @@ pub fn dof_text(dof: usize, ok: bool) -> String {
 }
 
 /// The line type new curves get in the sketch being edited (0 normal, 1 construction,
-/// 2 centerline); a new sketch starts normal.
+/// 2 centerline, 3 infinite); a new sketch starts normal.
 pub fn mode(app: &SolveApp) -> u8 {
     let (m, sk) = MODE.with(Cell::get);
     if sk.is_some() && sk == app.session.active_sketch { m } else { 0 }
@@ -95,6 +95,7 @@ pub fn set_mode(app: &mut SolveApp, m: u8) {
     let hint = match m {
         1 => "Construction mode: new curves are construction geometry (X or the palette's Normal to leave)",
         2 => "Centerline mode: new lines are centerlines",
+        3 => "Infinite mode: new lines are infinite construction lines",
         _ => "Normal mode",
     };
     app.set_status(hint, false);
@@ -124,7 +125,7 @@ pub fn apply_mode(app: &mut SolveApp, id: &str, result: &Value) {
     if curves.is_empty() {
         return;
     }
-    let lines: Vec<String> = if m == 2 {
+    let lines: Vec<String> = if m == 2 || m == 3 {
         let st = app.session.model.state();
         let Some(ss) = app.session.active_sketch.and_then(|s| st.sketch(s)) else { return };
         curves
@@ -144,7 +145,8 @@ pub fn apply_mode(app: &mut SolveApp, id: &str, result: &Value) {
     // Part of the drawing's own undo step.
     let depth = app.session.undo.len();
     if !lines.is_empty() {
-        let _ = app.session.execute("sketch.centerline", &json!({ "curves": lines, "value": true }));
+        let line_type = if m == 3 { "sketch.infinite" } else { "sketch.centerline" };
+        let _ = app.session.execute(line_type, &json!({ "curves": lines, "value": true }));
     }
     if !rest.is_empty() {
         let _ = app.session.execute("sketch.construction", &json!({ "curves": rest, "value": true }));
@@ -158,9 +160,10 @@ fn selected_curves(app: &SolveApp) -> Vec<String> {
 }
 
 /// The line types among the selected curves of the active sketch, indexed like the palette's
-/// buttons (0 normal, 1 construction, 2 centerline); a mixed selection marks each of its types.
-fn selected_linetypes(app: &SolveApp) -> [bool; 3] {
-    let mut types = [false; 3];
+/// buttons (0 normal, 1 construction, 2 centerline, 3 infinite); a mixed selection marks each of
+/// its types.
+fn selected_linetypes(app: &SolveApp) -> [bool; 4] {
+    let mut types = [false; 4];
     let curves = selected_curves(app);
     if curves.is_empty() {
         return types;
@@ -168,7 +171,9 @@ fn selected_linetypes(app: &SolveApp) -> [bool; 3] {
     let st = app.session.world_state();
     let Some(ss) = app.session.active_sketch.and_then(|s| st.sketch(s)) else { return types };
     for c in curves.iter().filter_map(|id| ss.sketch.curve_index(id).and_then(|i| ss.sketch.curves.get(i))) {
-        let k = if c.centerline {
+        let k = if c.infinite {
+            3
+        } else if c.centerline {
             2
         } else if c.construction {
             1
@@ -182,7 +187,8 @@ fn selected_linetypes(app: &SolveApp) -> [bool; 3] {
     types
 }
 
-/// Set the line type of the selected curves: 0 normal, 1 construction, 2 centerline (lines).
+/// Set the line type of the selected curves: 0 normal, 1 construction, 2 centerline, 3 infinite
+/// (the last two for lines).
 pub fn set_linetype(app: &mut SolveApp, kind: u8) {
     let curves = selected_curves(app);
     if curves.is_empty() {
@@ -210,7 +216,14 @@ pub fn set_linetype(app: &mut SolveApp, kind: u8) {
             }
             app.run("sketch.construction", json!({ "curves": curves, "value": false }))
         }
-        1 => app.run("sketch.construction", json!({ "curves": curves, "value": true })),
+        1 => {
+            if !lines.is_empty() {
+                let _ = app.run("sketch.infinite", json!({ "curves": lines, "value": false }));
+            }
+            app.run("sketch.construction", json!({ "curves": curves, "value": true }))
+        }
+        3 if lines.is_empty() => Err("an infinite line must be a line".to_string()),
+        3 => app.run("sketch.infinite", json!({ "curves": lines, "value": true })),
         _ if lines.is_empty() => Err("a centerline must be a line".to_string()),
         _ => app.run("sketch.centerline", json!({ "curves": lines, "value": true })),
     };
@@ -430,7 +443,7 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                             // Centerline need a round dot and a clear gap to read apart. 30 by
                             // 20 px left too little square room for either.
                             let btn_w = 36.0;
-                            let group_w = 3.0 * btn_w + 2.0 * ui.spacing().item_spacing.x;
+                            let group_w = 4.0 * btn_w + 3.0 * ui.spacing().item_spacing.x;
                             crate::frame::right_aligned(ui, width, group_w, |ui| {
                                 ui.horizontal(|ui| {
                                     // With a selection: its line type, and a click converts it;
@@ -442,10 +455,14 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                                     // construction orange. Normal's icon never changes with the
                                     // accent colour. It stays neutral in every case.
                                     let current = mode(app);
-                                    for (i, (label, icon, tip)) in
-                                        [("—", "normal", "Normal"), ("- -", "construction", "Construction"), ("-·-", "centerline", "Centerline")]
-                                            .iter()
-                                            .enumerate()
+                                    for (i, (label, icon, tip)) in [
+                                        ("—", "normal", "Normal"),
+                                        ("- -", "construction", "Construction"),
+                                        ("-·-", "centerline", "Centerline"),
+                                        ("↔", "infinite", "Infinite"),
+                                    ]
+                                    .iter()
+                                    .enumerate()
                                     {
                                         let on = if has_curves { selected_types.get(i).copied().unwrap_or(false) } else { current == i as u8 };
                                         let tr_tip = app.tr_label(tip);
diff --git a/crates/ui-egui/src/tools.rs b/crates/ui-egui/src/tools.rs
index 8a8e22ae..cd23fc7b 100644
--- a/crates/ui-egui/src/tools.rs
+++ b/crates/ui-egui/src/tools.rs
@@ -533,7 +533,9 @@ pub(crate) fn paint_preview_curve(
     mode: u8,
 ) {
     for curve in &mut sketch.curves {
-        curve.centerline = mode == 2 && matches!(curve.kind, solidcraft_engine::sketch::CurveKind::Line { .. });
+        let line = matches!(curve.kind, solidcraft_engine::sketch::CurveKind::Line { .. });
+        curve.centerline = mode == 2 && line;
+        curve.infinite = mode == 3 && line;
         curve.construction = mode != 0 && !curve.centerline;
     }
     paint_curve(&sketch, 0, plane, painter, proj, stroke, &mut solidcraft_engine::render::SamplingBudget::new(4096, 65_536));
diff --git a/crates/ui-egui/src/viewport.rs b/crates/ui-egui/src/viewport.rs
index f4344154..583915d3 100644
--- a/crates/ui-egui/src/viewport.rs
+++ b/crates/ui-egui/src/viewport.rs
@@ -807,7 +807,8 @@ fn ground_shadow(sc: &mut GpuScene, app: &SolveApp, st: &std::sync::Arc<solidcra
 /// Sketch line types share the same world-space dash phase in the scene and overlays.
 pub(crate) fn sketch_segments(pts: &[Vec3], mode: u8, unit: f64) -> Vec<(Vec3, Vec3)> {
     match mode {
-        1 => dashes(pts, &[0.3 * unit, 0.2 * unit]),
+        // Construction, and infinite lines (#1359), which are construction too.
+        1 | 3 => dashes(pts, &[0.3 * unit, 0.2 * unit]),
         2 => dashes(pts, &[0.6 * unit, 0.15 * unit, 0.05 * unit, 0.15 * unit]),
         _ => pts.windows(2).map(|w| (w[0], w[1])).collect(),
     }