Tabs: Snap to Unit, or Shift held, puts a tab stop on the ruler's nearest tick

A tab stop placed or dragged on the Tabs panel's ruler always landed
at a fractional position, and the ruler was numbered in inches with a
tick every 18 pt whatever the document's units. The reference app
snaps stops to the ruler units with Snap To Unit in the panel menu or
with Shift held during a drag, and numbers its tab ruler in the
document's units.

The ruler is now numbered in the General unit with the canvas rulers'
label step at the ruler's scale, and has a tick every tenth of a step.
The panel menu has Snap to Unit, checked while it is on. With it on,
or with Shift held, a stop placed by a click or dragged on the ruler
lands on the nearest tick, through Unit::snap_to_ruler_tick as a
Shift-dragged ruler guide does. A snapped click that lands on an
existing stop's tick selects that stop and records no undo step. The
stops still change through text.tabs.set. The reference app's page
names Shift for a drag only; Shift also snapping a click, and a
snapped click onto a stop's tick selecting that stop, are VectorCraft's
own rules.

Docs: docs/target-app-parity.md.

crates/ui-egui/src/i18n/bg.tsv

@@ -2952,6 +2952,7 @@
 	Snap to Pixel	Прилепване към пиксел
 	Precise Cursors	Прецизни курсори
 	Snap to Point	Прилепване към точка
+	Snap to Unit	Прилепване към мерна единица
 	Snapping Tolerance	Толеранс на прилепване
 	Social Square Post 1080×1080	Публикация за квадрат в социални мрежи 1080×1080
 	Soft Light	Мека светлина

crates/ui-egui/src/i18n/cs.tsv

@@ -1905,6 +1905,7 @@ lighting	Light	Světlo
 	Right-Justified Tab	Tabulátor zarovnaný doprava
 	Select a text object to set its tab stops.	Vyberte textový objekt, abyste mohli nastavit jeho zarážky tabulátoru.
 	Select text	Vyberte text
+	Snap to Unit	Přichytávat k jednotkám
 	+ Add Scale	+ Přidat měřítko
 	Add Selected Artwork as One Asset	Přidat vybranou kresbu jako jeden prvek
 	Export Settings	Nastavení exportu

crates/ui-egui/src/i18n/da.tsv

@@ -1734,6 +1734,7 @@ lighting	Light	Lys
 	Right-Justified Tab	Højrejusteret tabulator
 	Select a text object to set its tab stops.	Markér et tekstobjekt for at angive dets tabulatorstop.
 	Select text	Markér tekst
+	Snap to Unit	Fastgør til enhed
 	+ Add Scale	+ Tilføj skala
 	Add Selected Artwork as One Asset	Tilføj markeret grafik som ét aktiv
 	Export Settings	Eksportindstillinger

crates/ui-egui/src/i18n/de.tsv

@@ -2994,6 +2994,7 @@ effect	Points	Punkte
 	Snap to Pixel	An Pixel ausrichten
 	Precise Cursors	Präzise Cursor
 	Snap to Point	An Punkt ausrichten
+	Snap to Unit	An Einheit ausrichten
 	Snapping Tolerance	Einrasttoleranz
 	Social Square Post 1080×1080	Quadratischer Social-Media-Beitrag 1080×1080
 	Soft Light	Weiches Licht

crates/ui-egui/src/i18n/el.tsv

@@ -1833,6 +1833,7 @@
 	Snap to Grid	Προσκόλληση στο πλέγμα
 	Snap to Pixel	Προσκόλληση σε εικονοστοιχείο
 	Snap to Point	Προσκόλληση σε σημείο
+	Snap to Unit	Προσκόλληση σε μονάδα
 	Soft Light	Απαλό φως
 	Solid	Συνεχές
 	Spacing:	Απόσταση:

crates/ui-egui/src/i18n/es.tsv

@@ -3011,6 +3011,7 @@
 	Snap to Pixel	Ajustar a píxel
 	Precise Cursors	Cursores precisos
 	Snap to Point	Ajustar a punto
+	Snap to Unit	Ajustar a unidad
 	Snapping Tolerance	Tolerancia de ajuste
 	Social Square Post 1080×1080	Publicación cuadrada para redes 1080×1080
 	Soft Light	Luz suave

crates/ui-egui/src/i18n/fi.tsv

@@ -1868,6 +1868,7 @@ lighting	Light	Valo
 	Right-Justified Tab	Oikealle tasattu sarkain
 	Select a text object to set its tab stops.	Valitse tekstiobjekti, jonka sarkainkohdat haluat asettaa.
 	Select text	Valitse teksti
+	Snap to Unit	Kohdista yksikköön
 	+ Add Scale	+ Lisää skaalaus
 	Add Selected Artwork as One Asset	Lisää valittu kuvitus yhtenä resurssina
 	Export Settings	Vientiasetukset

crates/ui-egui/src/i18n/fr.tsv

@@ -3008,6 +3008,7 @@
 	Snap to Pixel	Magnétisme du pixel
 	Precise Cursors	Pointeurs précis
 	Snap to Point	Magnétisme du point
+	Snap to Unit	Magnétisme de l’unité
 	Snapping Tolerance	Tolérance du magnétisme
 	Social Square Post 1080×1080	Publication carrée pour réseaux sociaux 1080×1080
 	Soft Light	Lumière tamisée

crates/ui-egui/src/i18n/hu.tsv

@@ -3631,6 +3631,7 @@ mark-style	Japanese	Japán
 	Snap to Pixel	Illesztés pixelhez
 	Precise Cursors	Pontos kurzorok
 	Snap to Point	Illesztés pontra
+	Snap to Unit	Illesztés mértékegységhez
 	Snapping Tolerance	Illesztési tűrés
 	Social	Közösségi média
 	Social Landscape Post 1200×630	Fekvő közösségimédia-bejegyzés 1200×630

crates/ui-egui/src/i18n/id.tsv

@@ -1734,6 +1734,7 @@ lighting	Light	Cahaya
 	Right-Justified Tab	Tab rata kanan
 	Select a text object to set its tab stops.	Pilih objek teks untuk mengatur tab stop-nya.
 	Select text	Pilih teks
+	Snap to Unit	Snap ke satuan
 	+ Add Scale	+ Tambah skala
 	Add Selected Artwork as One Asset	Tambahkan karya seni terpilih sebagai satu aset
 	Export Settings	Pengaturan ekspor

crates/ui-egui/src/i18n/it.tsv

@@ -3044,6 +3044,7 @@ mark-style	Japanese	Giapponese
 	Snap to Pixel	Aggancia ai pixel
 	Precise Cursors	Cursori di precisione
 	Snap to Point	Aggancia al punto
+	Snap to Unit	Aggancia all'unità
 	Snapping Tolerance	Tolleranza di aggancio
 	Social Square Post 1080×1080	Post quadrato per social 1080×1080
 	Soft Light	Luce soffusa

crates/ui-egui/src/i18n/ja.tsv

@@ -2821,6 +2821,7 @@ mark-style	Japanese	日本式
 	Source Image	元画像
 	Image Trace View	画像トレースの表示
 	Snap Options	スナップオプション
+	Snap to Unit	単位にスナップ
 	Snapping Tolerance	スナップ許容値
 	Soft Light	ソフトライト
 	Solid	実線

crates/ui-egui/src/i18n/ka.tsv

@@ -1635,6 +1635,7 @@ lighting	Light	სინათლე
 	Right-Justified Tab	მარჯვნივ სწორებული ტაბულაცია
 	Select a text object to set its tab stops.	ტაბულაციის გაჩერებების დასაყენებლად მონიშნეთ ტექსტური ობიექტი.
 	Select text	მონიშნეთ ტექსტი
+	Snap to Unit	ერთეულზე მიბმა
 	+ Add Scale	+ მასშტაბის დამატება
 	Add Selected Artwork as One Asset	მონიშნული ნახატის დამატება ერთ რესურსად
 	Export Settings	ექსპორტის პარამეტრები

crates/ui-egui/src/i18n/ko.tsv

@@ -2815,6 +2815,7 @@ mark-style	Japanese	일본식
 	Source Image	원본 이미지
 	Image Trace View	이미지 추적 보기
 	Snap Options	스냅 옵션
+	Snap to Unit	단위에 스냅
 	Snapping Tolerance	스냅 허용 오차
 	Soft Light	소프트 라이트
 	Solid	단색

crates/ui-egui/src/i18n/nl.tsv

@@ -1748,6 +1748,7 @@ lighting	Light	Licht
 	Right-Justified Tab	Rechts uitgelijnde tabstop
 	Select a text object to set its tab stops.	Selecteer een tekstobject om de tabstops in te stellen.
 	Select text	Selecteer tekst
+	Snap to Unit	Uitlijnen op eenheid
 	+ Add Scale	+ Schaal toevoegen
 	Add Selected Artwork as One Asset	Geselecteerde artwork als één asset toevoegen
 	Export Settings	Exportinstellingen

crates/ui-egui/src/i18n/pl.tsv

@@ -1824,6 +1824,7 @@ lighting	Light	Światło
 	Right-Justified Tab	Tabulator prawy
 	Select a text object to set its tab stops.	Zaznacz obiekt tekstowy, aby ustawić jego tabulatory.
 	Select text	Zaznacz tekst
+	Snap to Unit	Przyciągaj do jednostek
 	+ Add Scale	+ Dodaj skalę
 	Add Selected Artwork as One Asset	Dodaj zaznaczoną kompozycję jako jeden zasób
 	Export Settings	Ustawienia eksportu

crates/ui-egui/src/i18n/pt-br.tsv

@@ -2501,6 +2501,7 @@
 	Snap to Pixel	Ajustar ao Pixel
 	Precise Cursors	Cursores Precisos
 	Snap to Point	Ajustar ao Ponto
+	Snap to Unit	Ajustar à Unidade
 	Snapping Tolerance	Tolerância de Ajuste
 	Soft Light	Luz Suave
 	Solid	Sólido

crates/ui-egui/src/i18n/ru.tsv

@@ -3040,6 +3040,7 @@ mark-style	Japanese	Японский
 	Snap to Pixel	Привязать к пикселю
 	Precise Cursors	Точные курсоры
 	Snap to Point	Привязать к точке
+	Snap to Unit	Привязать к единицам
 	Snapping Tolerance	Допуск привязки
 	Social Square Post 1080×1080	Квадратный пост для соцсетей 1080×1080
 	Soft Light	Мягкий свет

crates/ui-egui/src/i18n/sv.tsv

@@ -2952,6 +2952,7 @@
 	Snap to Pixel	Fäst mot pixel
 	Precise Cursors	Exakta markörer
 	Snap to Point	Fäst mot punkt
+	Snap to Unit	Fäst mot enhet
 	Snapping Tolerance	Fästtolerans
 	Social Square Post 1080×1080	Kvadratiskt inlägg för sociala medier 1080×1080
 	Soft Light	Mjukt ljus

crates/ui-egui/src/i18n/uk.tsv

@@ -3037,6 +3037,7 @@ effect	Points	Точки
 	Snap to Pixel	Прив’язати до пікселя
 	Precise Cursors	Точні курсори
 	Snap to Point	Прив’язати до точки
+	Snap to Unit	Прив’язати до одиниць
 	Snapping Tolerance	Допуск прив’язки
 	Social Square Post 1080×1080	Квадратний пост для соцмереж 1080×1080
 	Soft Light	М’яке світло

crates/ui-egui/src/i18n/vi.tsv

@@ -3365,6 +3365,7 @@ mark-style	Japanese	Kiểu Nhật
 	Snap to Pixel	Hút vào pixel
 	Precise Cursors	Con trỏ chính xác
 	Snap to Point	Hút vào điểm
+	Snap to Unit	Hút vào đơn vị
 	Snapping Tolerance	Dung sai hút
 	Social Square Post 1080×1080	Bài đăng vuông mạng xã hội 1080×1080
 	Soft	Mềm

crates/ui-egui/src/i18n/zh-hans.tsv

@@ -2894,6 +2894,7 @@ mark-style	Japanese	日式
 	Snap to Pixel	对齐像素
 	Precise Cursors	精确光标
 	Snap to Point	对齐点
+	Snap to Unit	对齐单位
 	Snapping Tolerance	吸附容差
 	Social Square Post 1080×1080	社交方形帖子 1080×1080
 	Soft Light	柔光

crates/ui-egui/src/i18n/zh-hant.tsv

@@ -3760,6 +3760,7 @@ mark-style	Japanese	日式
 	Snap to Pixel	靠齊像素
 	Precise Cursors	精確游標
 	Snap to Point	靠齊控制點
+	Snap to Unit	靠齊單位
 	Snapping Tolerance	靠齊容許度
 	Social Square Post 1080×1080	社群正方形貼文 1080×1080
 	Soft Light	柔光

crates/ui-egui/src/panels/tabs.rs

@@ -1,5 +1,7 @@
-//! Tabs panel: tab-stop alignment, X position, leader and "align on", and a ruler on which a click
-//! adds a stop, dragging moves one and dragging it off the ruler deletes it.
+//! Tabs panel: tab-stop alignment, X position, leader and "align on", and a ruler numbered in the
+//! General unit on which a click adds a stop, dragging moves one and dragging it off the ruler
+//! deletes it. With Snap to Unit on, or Shift held, a stop placed or dragged on the ruler lands on
+//! the ruler's nearest tick.
 
 use egui::{Sense, Stroke, Ui, vec2};
 use serde_json::{Value, json};
@@ -175,22 +177,33 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
     let scale = (w as f64 - 8.0) / span;
     let to_x = |pt: f64| rect.left() + 4.0 + (pt * scale) as f32;
     let to_pt = |x: f32| (((x - rect.left() - 4.0) as f64) / scale).max(0.0);
-    let mut tick = 0.0;
-    while tick <= span + 1e-6 {
-        let x = to_x(tick);
-        let major = (tick / 36.0).round() * 36.0 == tick && ((tick / 36.0) as i64) % 2 == 0;
-        p.line_segment([egui::pos2(x, rect.bottom() - if major { 10.0 } else { 5.0 }), egui::pos2(x, rect.bottom())], Stroke::new(1.0, t.text_dim));
-        if major {
+    // The ruler is numbered in the General unit, with the canvas rulers' label step at its scale
+    // and a tick every tenth of a step.
+    let unit = app.session.general_unit();
+    let step = unit.ruler_step(scale);
+    let minor = step / 10.0;
+    for k in 0..=(unit.from_pt(span) / minor + 1e-6).floor() as usize {
+        let v = k as f64 * minor;
+        let x = to_x(unit.to_pt(v));
+        let len = match k % 10 {
+            0 => 10.0,
+            5 => 7.0,
+            _ => 4.0,
+        };
+        p.line_segment([egui::pos2(x, rect.bottom() - len), egui::pos2(x, rect.bottom())], Stroke::new(1.0, t.text_dim));
+        if k % 10 == 0 {
             p.text(
                 egui::pos2(x + 2.0, rect.top() + 1.0),
                 egui::Align2::LEFT_TOP,
-                format!("{}", (tick / 72.0 * 100.0).round() / 100.0),
+                crate::canvas::ruler_label(v, step),
                 egui::FontId::proportional(9.0),
                 t.text_dim,
             );
         }
-        tick += 18.0;
     }
+    // Snap to Unit, or Shift held: a stop placed or dragged on the ruler lands on its nearest tick.
+    let snap = pstate::<bool>(ui.ctx(), "tabs-snap") || ui.input(|i| i.modifiers.shift);
+    let place = |x: f32| if snap { unit.snap_to_ruler_tick(to_pt(x), scale).max(0.0) } else { to_pt(x) };
     for (i, s) in stops.iter().enumerate() {
         let x = to_x(s.position);
         let c = if i == sel { t.accent } else { t.text };
@@ -230,7 +243,7 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
                 // Dragged off the ruler: delete.
                 stops.remove(i);
             } else {
-                stops[i].position = to_pt(pos.x);
+                stops[i].position = place(pos.x);
                 stops.sort_by(|a, b| a.position.total_cmp(&b.position));
             }
             apply(app, &stops);
@@ -241,12 +254,18 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
         match hit_at(pos.x) {
             Some(i) => set_pstate(ui.ctx(), "tabs-sel", i),
             None => {
-                let position = to_pt(pos.x);
-                stops.push(TabStop { position, align: ALIGNS[align_idx.min(3)].0, leader: String::new(), align_on: '.' });
-                stops.sort_by(|a, b| a.position.total_cmp(&b.position));
+                let position = place(pos.x);
+                // A snapped click onto a stop's tick selects that stop and records no undo step.
+                let added = !stops.iter().any(|s| s.position == position);
+                if added {
+                    stops.push(TabStop { position, align: ALIGNS[align_idx.min(3)].0, leader: String::new(), align_on: '.' });
+                    stops.sort_by(|a, b| a.position.total_cmp(&b.position));
+                }
                 let i = stops.iter().position(|s| s.position == position).unwrap_or(0);
                 set_pstate(ui.ctx(), "tabs-sel", i);
-                apply(app, &stops);
+                if added {
+                    apply(app, &stops);
+                }
             }
         }
     }
@@ -256,6 +275,7 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
 pub fn menu(app: &mut VectorcraftApp, ui: &mut Ui) {
     let Some((_, para, _)) = current_para(app) else {
         ui.add_enabled(false, egui::Button::new(tl!("Select text")).frame(false));
+        snap_to_unit_item(ui);
         return;
     };
     let mut stops = para.tabs;
@@ -290,6 +310,17 @@ pub fn menu(app: &mut VectorcraftApp, ui: &mut Ui) {
         }
         apply(app, &stops);
     }
+    snap_to_unit_item(ui);
+}
+
+/// The panel menu's Snap to Unit, checked while it is on: stops placed or dragged on the ruler land
+/// on its nearest tick.
+fn snap_to_unit_item(ui: &mut Ui) {
+    ui.separator();
+    let on: bool = pstate(ui.ctx(), "tabs-snap");
+    if menu_item(ui, tl!("Snap to Unit"), true, on) {
+        set_pstate(ui.ctx(), "tabs-snap", !on);
+    }
 }
 
 #[cfg(test)]
@@ -405,9 +436,9 @@ mod tests {
         };
         let labels = frame(&mut app, vec![]);
         let at = |label: &str| labels.iter().find(|(t, _)| t == label).unwrap_or_else(|| panic!("{label} in {labels:?}")).1;
-        // A label sits 2 px right of its tick: 0 and 1 inch give the ruler's scale.
-        let (zero, inch) = (at("0"), at("1"));
-        let to_x = |pt: f32| zero.x - 2.0 + pt * (inch.x - zero.x) / 72.0;
+        // A label sits 2 px right of its tick: 0 and 100 pt give the ruler's scale.
+        let (zero, hundred) = (at("0"), at("100"));
+        let to_x = |pt: f32| zero.x - 2.0 + pt * (hundred.x - zero.x) / 100.0;
         let y = zero.y + 15.0;
         let button = |pos, pressed| egui::Event::PointerButton { pos, button: egui::PointerButton::Primary, pressed, modifiers: Default::default() };
         // Press 3 px right of the stop, then drag right; egui starts the drag 6+ px from the press.
@@ -425,6 +456,148 @@ mod tests {
         assert!((moved_to - end.x).abs() < 1.0, "stop at {moved_to}, released at {}", end.x);
     }
 
+    /// One frame of the panel with `events`, Shift held when `shift` → the ruler's labels (9 pt) and
+    /// where they are painted, left to right.
+    fn ruler_frame(ctx: &egui::Context, app: &mut VectorcraftApp, events: Vec<egui::Event>, shift: bool) -> Vec<(String, egui::Pos2)> {
+        let screen = egui::Rect::from_min_size(egui::Pos2::ZERO, vec2(400.0, 400.0));
+        let modifiers = if shift { egui::Modifiers::SHIFT } else { egui::Modifiers::NONE };
+        let events = std::iter::once(egui::Event::ModifiersChanged(modifiers)).chain(events).collect();
+        let mut out = ctx.run_ui(egui::RawInput { screen_rect: Some(screen), events, ..Default::default() }, |ui| show(app, ui));
+        out.textures_delta.clear();
+        out.shapes
+            .iter()
+            .filter_map(|c| match &c.shape {
+                egui::Shape::Text(t) if t.galley.job.sections.first().is_some_and(|s| s.format.font_id.size == 9.0) => {
+                    Some((t.galley.text().to_string(), t.pos))
+                }
+                _ => None,
+            })
+            .collect()
+    }
+
+    /// The labels the ruler shows in a frame of [`ruler_frame`].
+    fn ruler_labels(ctx: &egui::Context, app: &mut VectorcraftApp) -> Vec<String> {
+        ruler_frame(ctx, app, vec![], false).into_iter().map(|(t, _)| t).collect()
+    }
+
+    /// Press on the ruler at `from` (points), move to `to` and release there, Shift held when
+    /// `shift`. A label sits 2 px right of its tick, so the first two labels give the ruler's scale.
+    fn drag_on_ruler(ctx: &egui::Context, app: &mut VectorcraftApp, from: f64, to: f64, shift: bool) {
+        let labels = ruler_frame(ctx, app, vec![], shift);
+        let [(zero, a), (next, b), ..] = &labels[..] else { panic!("ruler labels {labels:?}") };
+        assert_eq!(zero, "0");
+        let per = next.parse::<f64>().unwrap() * app.session.general_unit().points();
+        let x = |pt: f64| a.x - 2.0 + (pt / per) as f32 * (b.x - a.x);
+        let y = a.y + 15.0;
+        let button = |pos, pressed| egui::Event::PointerButton { pos, button: egui::PointerButton::Primary, pressed, modifiers: Default::default() };
+        let (press, end) = (egui::pos2(x(from), y), egui::pos2(x(to), y));
+        ruler_frame(ctx, app, vec![egui::Event::PointerMoved(press), button(press, true)], shift);
+        if press != end {
+            for f in [0.25, 0.5, 1.0] {
+                ruler_frame(ctx, app, vec![egui::Event::PointerMoved(press.lerp(end, f))], shift);
+            }
+        }
+        ruler_frame(ctx, app, vec![button(end, false)], shift);
+        ruler_frame(ctx, app, vec![], shift);
+    }
+
+    /// The ruler is numbered in the General unit (the document's units), with labels at least 50 px
+    /// apart as on the canvas rulers.
+    #[test]
+    fn ruler_is_numbered_in_the_general_unit() {
+        let mut app = VectorcraftApp::new(Session::new(), Default::default());
+        app.session.execute("file.new", &json!({})).unwrap();
+        app.session.execute("text.create", &json!({"x": 10, "y": 20, "text": "a	b", "area": {"width": 300, "height": 100}})).unwrap();
+        let ctx = egui::Context::default();
+        assert_eq!(ruler_labels(&ctx, &mut app), ["0", "50", "100", "150", "200", "250", "300"]);
+        app.session.execute("document.setUnits", &json!({"units": "in"})).unwrap();
+        assert_eq!(ruler_labels(&ctx, &mut app), ["0", "1", "2", "3", "4"]);
+        app.session.execute("document.setUnits", &json!({"units": "mm"})).unwrap();
+        assert_eq!(ruler_labels(&ctx, &mut app), ["0", "25", "50", "75", "100"]);
+    }
+
+    /// Snap to Unit, chosen from the panel menu with or without text selected, puts a stop dragged
+    /// or placed by a click on the ruler on its nearest tick (every 5 pt on this points ruler).
+    /// Chosen again, it turns off.
+    #[test]
+    fn snap_to_unit_puts_dragged_and_clicked_stops_on_the_nearest_tick() {
+        let mut app = VectorcraftApp::new(Session::new(), Default::default());
+        app.session.execute("file.new", &json!({})).unwrap();
+        let ctx = egui::Context::default();
+        let row = |app: &mut VectorcraftApp| {
+            let rows = menu_frame(&ctx, app, vec![]);
+            rows.into_iter().find(|(t, _)| t.trim_start_matches(['✓', ' ']) == "Snap to Unit").unwrap_or_else(|| panic!("Snap to Unit"))
+        };
+        choose(&ctx, &mut app, "Snap to Unit");
+        assert_eq!(row(&mut app).0, "✓ Snap to Unit");
+        app.session.execute("text.create", &json!({"x": 10, "y": 20, "text": "a	b", "area": {"width": 300, "height": 100}})).unwrap();
+        app.session.execute("text.tabs.set", &json!({"stops": [{"position": 50}]})).unwrap();
+        assert_eq!(ruler_labels(&ctx, &mut app)[..2], ["0", "50"]);
+        let positions = |app: &VectorcraftApp| current(app).unwrap().1.iter().map(|s| s.position).collect::<Vec<_>>();
+        drag_on_ruler(&ctx, &mut app, 50.0, 13.4, false);
+        assert_eq!(positions(&app), [15.0]);
+        // A click at 33.4 pt adds a stop at 35.
+        drag_on_ruler(&ctx, &mut app, 33.4, 33.4, false);
+        assert_eq!(positions(&app), [15.0, 35.0]);
+        assert_eq!(row(&mut app).0, "✓ Snap to Unit");
+        // Chosen again, Snap to Unit turns off and the stop stays where it is dropped.
+        let at = row(&mut app).1.center();
+        let button = |pressed| egui::Event::PointerButton { pos: at, button: egui::PointerButton::Primary, pressed, modifiers: Default::default() };
+        menu_frame(&ctx, &mut app, vec![egui::Event::PointerMoved(at), button(true)]);
+        menu_frame(&ctx, &mut app, vec![button(false)]);
+        assert_eq!(row(&mut app).0.trim_start(), "Snap to Unit");
+        drag_on_ruler(&ctx, &mut app, 35.0, 23.4, false);
+        let p = positions(&app);
+        assert!((p[1] - 23.4).abs() < 1.0 && p[1] != 25.0, "{p:?}");
+    }
+
+    /// A snapped click more than 5 px from a stop, nearest to the tick that stop is on, selects the
+    /// stop and records no undo step. On this millimeters ruler the ticks are 2.5 mm (7.09 pt, 12.4
+    /// px) apart.
+    #[test]
+    fn a_snapped_click_onto_a_stop_selects_it() {
+        let mut app = VectorcraftApp::new(Session::new(), Default::default());
+        app.session.execute("file.new", &json!({})).unwrap();
+        app.session.execute("document.setUnits", &json!({"units": "mm"})).unwrap();
+        app.session.execute("text.create", &json!({"x": 10, "y": 20, "text": "a	b", "area": {"width": 224, "height": 100}})).unwrap();
+        app.session.execute("text.tabs.set", &json!({"stops": [{"position": 100}]})).unwrap();
+        let ctx = egui::Context::default();
+        assert_eq!(ruler_labels(&ctx, &mut app), ["0", "25", "50", "75"]);
+        set_pstate(&ctx, "tabs-snap", true);
+        let tick = 2.5 * 72.0 / 25.4;
+        drag_on_ruler(&ctx, &mut app, 28.4, 28.4, false);
+        let positions = |app: &VectorcraftApp| current(app).unwrap().1.iter().map(|s| s.position).collect::<Vec<_>>();
+        assert_eq!(positions(&app).len(), 2);
+        assert!((positions(&app)[0] - 4.0 * tick).abs() < 1e-9, "{:?}", positions(&app));
+        set_pstate(&ctx, "tabs-sel", 1usize);
+        let steps = app.session.active().unwrap().history.undo.len();
+        // The click is 3.3 pt (5.8 px) right of the stop, past the 5 px hit tolerance and nearest its tick.
+        drag_on_ruler(&ctx, &mut app, 4.0 * tick + 3.3, 4.0 * tick + 3.3, false);
+        assert_eq!(positions(&app).len(), 2, "{:?}", positions(&app));
+        assert_eq!(pstate::<usize>(&ctx, "tabs-sel"), 0);
+        assert_eq!(app.session.active().unwrap().history.undo.len(), steps, "no undo step");
+    }
+
+    /// With Snap to Unit off, Shift held during the drag puts the stop on the ruler's nearest tick.
+    #[test]
+    fn shift_drag_snaps_a_stop_to_the_nearest_tick() {
+        let mut app = VectorcraftApp::new(Session::new(), Default::default());
+        app.session.execute("file.new", &json!({})).unwrap();
+        app.session.execute("text.create", &json!({"x": 10, "y": 20, "text": "a	b", "area": {"width": 300, "height": 100}})).unwrap();
+        app.session.execute("text.tabs.set", &json!({"stops": [{"position": 50}]})).unwrap();
+        let ctx = egui::Context::default();
+        let position = |app: &VectorcraftApp| current(app).unwrap().1[0].position;
+        drag_on_ruler(&ctx, &mut app, 50.0, 13.4, false);
+        assert!((position(&app) - 13.4).abs() < 1.0, "{}", position(&app));
+        let from = position(&app);
+        drag_on_ruler(&ctx, &mut app, from, 41.6, true);
+        assert_eq!(position(&app), 40.0);
+        // In an inches document the ticks are a tenth of an inch (7.2 pt) apart.
+        app.session.execute("document.setUnits", &json!({"units": "in"})).unwrap();
+        drag_on_ruler(&ctx, &mut app, 40.0, 100.0, true);
+        assert!((position(&app) - 100.8).abs() < 1e-9, "{}", position(&app));
+    }
+
     /// One frame of the panel menu with `events` → the rows' texts and where they are painted.
     fn menu_frame(ctx: &egui::Context, app: &mut VectorcraftApp, events: Vec<egui::Event>) -> Vec<(String, egui::Rect)> {
         let screen = egui::Rect::from_min_size(egui::Pos2::ZERO, vec2(320.0, 400.0));

docs/target-app-parity.md

@@ -633,6 +633,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   The frames of threaded text share one Optical Margin Alignment setting: turning it on or off in one frame sets it in every frame, threading frames gives the thread its first frame's setting, and frames taken out of a thread keep it.
   Type › Optical Margin Alignment (`type.opticalMarginAlignment`) hangs the punctuation and letter edges at the ends of a type object's lines outside its margins: at the left of left-aligned lines, at the right of right-aligned lines, and at both ends of centered and justified lines, in point and area type of left-to-right paragraphs. Quotation marks, hyphens, full stops and commas hang wholly; asterisks, tildes, ellipses, en and em dashes, colons and semicolons hang by half; and A, V, W, Y, v, w and y hang by a tenth of their width. It is off by default and saved with the type object.
   The Paragraph panel menu's Hyphenation… (`ui.hyphenation`, `text.setFormat {wordsLongerThan, afterFirst, beforeLast, hyphenLimit, hyphenationZone, hyphenateCapitalized}`) sets each paragraph's hyphenation limits: the shortest word that breaks, the fewest letters on each side of the hyphen, the most hyphenated lines in a row, a zone at the right edge where the Single-line Composer breaks before a word instead of hyphenating it, and whether capitalized words break. The defaults are words longer than 5 letters, 3 letters on each side, no limit or zone, and capitalized words hyphenated; paragraph styles and the native file keep the options.
+  The Tabs panel's ruler is numbered in the General unit with the canvas rulers' label steps and a tick every tenth of a step. With the panel menu's Snap to Unit on, or Shift held, a tab stop placed by a click on the ruler or dragged on it lands on the nearest tick (`text.tabs.set`).
   The Tabs panel menu's Clear All Tabs (`text.tabs.clear`), Delete Tab and Repeat Tab (`text.tabs.set`) can be chosen while the panel's ruler shows tab stops. Delete Tab removes the stop selected on the ruler. Repeat Tab replaces the stops after the selected stop with up to 20 copies of it. The copies are spaced by the selected stop's distance from the stop before it, or from the paragraph's left indent when it is the first stop, and no copy is placed at 2,000 pt or beyond. Clear All Tabs removes every stop, so the default stops every ½ inch apply again.
 - **Interface languages, vertical type:** Preferences ▸ User Interface (or VectorCraft ▸ Language on macOS) picks Automatic (the system locale) or a registered language, persisted as `interfaceLanguage`. Translations come from per-language catalogs (`i18n/*.tsv`): Traditional and Simplified Chinese, Japanese, German (every `de-*` locale), Spanish (neutral international Spanish, for every `es-*` locale), French (every `fr-*` locale), Italian (every `it-*` locale) Russian (every `ru-*` locale) and Ukrainian (every `uk-*` locale) cover the whole interface, Czech every menu label; Czech, German, Greek, Spanish, French, Hungarian, Italian, Japanese, Korean, Vietnamese, Polish, Brazilian Portuguese, Russian, Simplified Chinese, Traditional Chinese, Swedish, Ukrainian and Finnish also translate status-bar and error messages as they are shown (`@msg` templates, every message literal tested; the messages themselves stay English for agents); untranslated text stays English. Japanese and Chinese glyphs come from the craft-fonts build input (release builds) or the installed fonts, Czech, German, Spanish, French, Italian, Russian and Ukrainian ones from the bundled UI fonts. The web app follows the browser's languages (or `?lang=`). The Basic toolbar's Type button holds Vertical Type in its flyout. Vertical Type, Vertical Area Type and Vertical Type on a Path create vertical text (columns right to left, upright CJK glyphs and marks with `vert` alternates, Latin and longer numbers on their side, two- and three-digit numbers set as tate-chu-yoko in one em of the column (kept whole when justified), upright glyphs centred on the column (tracked or justified too), point type's anchor on the first column's centre line); kinsoku applies to horizontal and vertical Japanese (Paragraph panel › Kinsoku Set, `text.setFormat {kinsoku}`: Hard, the default, keeps closing marks, iteration marks, the prolonged sound mark and small kana off the line start and opening brackets off its end; Soft lets 々, ー and small kana start a line as JLREQ's level 3 rules do; None turns it off), and justified Japanese lines spread their room between characters (not inside Latin words); Paragraph panel › Mojikumi Set (shown with Preferences › Type › Show East Asian Options, `text.setFormat {mojikumi}`): None or Line-end Punctuation Half Width (new type's default, as in Illustrator; older documents and imported text keep None) shares one half-em space between consecutive punctuation, sets a closing mark ending a line half width and an opening bracket starting a line flush with its start (a paragraph's first line at its first-line indent, JIS X 4051's principle in JLREQ 3.1.5), and puts a quarter em between Japanese and Latin letters or digits; Paragraph panel menu › Top-to-Top / Bottom-to-Bottom Leading measures leading from em box top to top (area type's first line touches the frame) or from baseline to baseline (new type in a Japanese interface starts top to top, with em box centre character alignment, #432); Paragraph panel menu › Burasagari None / Regular / Force (`text.setFormat {burasagari: "none"|"standard"|"forced"}`) hangs an East Asian comma or full stop (、。，．､｡) ending an area type line outside it, Regular (new type's default) when it doesn't fit and Force always (the rest of the line then fills the measure; closing brackets and Latin punctuation don't hang); Character panel menu › Character Alignment (with the East Asian options; `text.setFormat` or `text.setRangeStyle {charAlign}`) lines characters smaller than the largest on their line up on the Roman baseline, the em box top/right, centre or bottom/left, or the ICF (ideographic character face) top/right or bottom/left, from the font's BASE table or, without one, the average ink box of some ideographs and kana; Proportional Metrics (the same menu; `{proportionalMetrics}`) sets full-width glyphs on the font's proportional widths (OpenType `palt`) in horizontal type and upright glyphs on its proportional heights (`vpal`, shaped top to bottom; fonts without vertical metrics keep their cells) in vertical type, and Line-end Punctuation Half Width takes nothing more off the punctuation they re-spaced; Type ▸ Type Orientation switches existing text; caret, selection, hit testing and arrow keys follow the writing direction. Vertical text exports to PDF as real text and to SVG as outlines. The interface is available in Polish (`pl`): menus, panels, dialogs and messages, with one/few/many plurals; AI-drafted, native review pending (#1498).
 - **Transparency:** opacity masks (clip/invert/disable/link), exported as SVG `<mask>` and PDF soft masks. The constant alpha soft mask that PDF and `.ai` export draw a CMYK page with transparency through is white, so applications that read every soft mask by its luminosity show the page too (#1359, part of #1286).