Type: an edit copies a threaded story only when it changes one of its frames

After every edit of a document with threaded text, each frame of a
drag included, threads::reflow copied the runs and the text object of
every frame of every thread, and then dropped the copies for each
thread whose frames the edit had not changed. With a story of 50,000
characters threaded through 8 frames with a style change at every word
(9,618 runs), moving another object took 7.8 ms in a release build,
and a frame of a drag took 7.2 ms.

reflow now copies a thread's story only when one of its frames
changed. The same move takes 0.013 ms, and the drag frame takes
0.012 ms. Threads with a changed frame re-flow as before, and the
documents and undo steps after edits are the same as before.

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

crates/engine/src/cmd/threads.rs

@@ -62,6 +62,12 @@ fn text_of(d: &Document, id: NodeId) -> Option<&TextObject> {
     }
 }
 
+#[cfg(test)]
+thread_local! {
+    /// Tests: how many threads' stories [`reflow`] copied on this thread.
+    static COPIED: std::cell::Cell<usize> = const { std::cell::Cell::new(0) };
+}
+
 /// Re-flow every thread with a frame that changed between `before` and `doc` (structural sharing
 /// makes "changed" a pointer comparison). Deleted frames' text flows into the remaining ones.
 pub(crate) fn reflow(before: &Document, doc: &mut Document) {
@@ -73,6 +79,16 @@ pub(crate) fn reflow(before: &Document, doc: &mut Document) {
             (None, None) => false,
             _ => true,
         });
+        // When no frame changed, the thread keeps its area-type frames and their text is not copied.
+        if !changed {
+            let live: Vec<NodeId> = thread.into_iter().filter(|id| text_of(doc, *id).is_some()).collect();
+            if live.len() >= 2 {
+                kept.push(live);
+            }
+            continue;
+        }
+        #[cfg(test)]
+        COPIED.with(|c| c.set(c.get() + 1));
         // The story: live frames' runs, plus the text of frames deleted by this edit.
         let mut story: Vec<TextRun> = vec![];
         let mut sources: Vec<TextObject> = vec![];
@@ -96,10 +112,7 @@ pub(crate) fn reflow(before: &Document, doc: &mut Document) {
                 (None, None) => {}
             }
         }
-        if !changed || live.is_empty() {
-            if live.len() >= 2 {
-                kept.push(live);
-            }
+        if live.is_empty() {
             continue;
         }
         let source_refs: Vec<&TextObject> = sources.iter().collect();
@@ -293,6 +306,113 @@ mod tests {
         assert_eq!(s.doc().unwrap().doc.text_threads.len(), 1);
     }
 
+    #[test]
+    fn edits_copy_only_the_stories_of_threads_whose_frames_changed() {
+        let (mut s, a, b) = setup();
+        s.execute("text.thread.create", &json!({})).unwrap();
+        let c = s.execute("text.create", &json!({"x": 10, "y": 200, "text": TEXT.repeat(2), "area": {"width": 150, "height": 40}})).unwrap()["id"]
+            .as_u64()
+            .unwrap();
+        let d = s.execute("shape.rectangle", &json!({"x": 200, "y": 220, "width": 150, "height": 60})).unwrap()["id"].as_u64().unwrap();
+        s.execute("select.set", &json!({"ids": [c, d]})).unwrap();
+        s.execute("text.thread.create", &json!({})).unwrap();
+        let texts = |s: &Session| [a, b, c, d].map(|id| text(s, id));
+        let copied = || COPIED.with(std::cell::Cell::get);
+        let (shown, start) = (texts(&s), copied());
+        // A rectangle drawn, moved and dragged beside the threads changes none of their frames.
+        let r = s.execute("shape.rectangle", &json!({"x": 400, "y": 250, "width": 20, "height": 20})).unwrap()["id"].as_u64().unwrap();
+        s.execute("select.set", &json!({"ids": [r]})).unwrap();
+        s.execute("object.move", &json!({"dx": 5, "dy": 0})).unwrap();
+        s.begin_interaction("Move").unwrap();
+        for dx in 1..=3 {
+            s.preview("object.move", &json!({"dx": dx, "dy": 0})).unwrap();
+        }
+        s.commit_interaction().unwrap();
+        assert_eq!(copied(), start, "edits beside the threads copied their stories");
+        assert_eq!(texts(&s), shown);
+        // Typing in the first thread copies and re-flows that thread's story only.
+        s.execute("text.editRange", &json!({"id": a, "start": 0, "end": 0, "insert": "NEW "})).unwrap();
+        assert_eq!(copied(), start + 1);
+        assert_eq!(format!("{}{}", text(&s, a), text(&s, b)), format!("NEW {}", TEXT.repeat(4)));
+        assert_eq!(texts(&s)[2..], shown[2..]);
+        let ids = |t: [u64; 2]| t.map(NodeId).to_vec();
+        assert_eq!(s.doc().unwrap().doc.text_threads, [ids([a, b]), ids([c, d])]);
+    }
+
+    /// Time edits of a story of 50,000 characters threaded through 8 frames, each frame wrapping
+    /// around an ellipse, with one style, a style change every 10 words and a style change at every
+    /// word: 21 characters typed into the first frame, then 200 moves of a rectangle below the
+    /// frames and 200 frames of a drag of it.
+    /// Run with `cargo test --release -p vectorcraft-engine --lib threaded_story_edit_timing -- --ignored --nocapture`.
+    #[test]
+    #[ignore = "manual timing probe; wall-clock performance depends on machine load"]
+    fn threaded_story_edit_timing() {
+        fn median(s: &mut Session, n: u32, f: &dyn Fn(&mut Session, f64)) -> f64 {
+            let mut ms: Vec<f64> = (1..=n)
+                .map(|i| {
+                    let start = std::time::Instant::now();
+                    f(s, f64::from(i));
+                    start.elapsed().as_secs_f64() * 1000.0
+                })
+                .collect();
+            ms.sort_by(f64::total_cmp);
+            ms[ms.len() / 2]
+        }
+        let story = format!("{}\n", TEXT.repeat(6).trim_end()).repeat(81)[..50_000].to_string();
+        for words in [0, 10, 1] {
+            let mut s = Session::new();
+            s.execute("file.new", &json!({"width": 4400, "height": 1200})).unwrap();
+            let area = json!({"width": 500, "height": 900});
+            let first = s.execute("text.create", &json!({"x": 20, "y": 20, "text": story, "size": 9, "area": area})).unwrap()["id"].as_u64().unwrap();
+            if words > 0 {
+                s.edit("Restyle", |d, _| {
+                    let Some(NodeKind::Text(t)) = d.node_mut(NodeId(first)).map(|n| &mut n.kind) else { panic!("not text") };
+                    let style = t.runs[0].style.clone();
+                    let pieces: Vec<&str> = story.split_inclusive([' ', '\n']).collect();
+                    t.runs = pieces
+                        .chunks(words)
+                        .enumerate()
+                        .map(|(i, c)| {
+                            let mut run = TextRun::new(c.concat(), style.clone());
+                            run.style.underline = i % 2 == 1;
+                            run
+                        })
+                        .collect();
+                    refresh_bounds(t);
+                    Ok(())
+                })
+                .unwrap();
+            }
+            let mut ids = vec![first];
+            for k in 1..8 {
+                let frame = json!({"x": 20 + k * 540, "y": 20, "width": 500, "height": 900});
+                ids.push(s.execute("shape.rectangle", &frame).unwrap()["id"].as_u64().unwrap());
+            }
+            s.execute("select.set", &json!({"ids": ids})).unwrap();
+            s.execute("text.thread.create", &json!({})).unwrap();
+            for k in 0..8 {
+                let e = s.execute("shape.ellipse", &json!({"x": 195 + k * 540, "y": 395, "width": 150, "height": 150})).unwrap()["id"].clone();
+                s.execute("object.textWrap.make", &json!({"ids": [e]})).unwrap();
+            }
+            let r = s.execute("shape.rectangle", &json!({"x": 20, "y": 1100, "width": 30, "height": 30})).unwrap()["id"].clone();
+            s.execute("select.set", &json!({"ids": [r]})).unwrap();
+            let typing = median(&mut s, 21, &|s, _| {
+                s.execute("text.editRange", &json!({"id": first, "start": 0, "end": 0, "insert": "x"})).unwrap();
+            });
+            let edit = median(&mut s, 200, &|s, _| {
+                s.execute("object.move", &json!({"dx": 0.5, "dy": 0})).unwrap();
+            });
+            s.begin_interaction("Move").unwrap();
+            let frame = median(&mut s, 200, &|s, i| {
+                s.preview("object.move", &json!({"dx": i * 0.25, "dy": 0})).unwrap();
+            });
+            s.commit_interaction().unwrap();
+            let d = &s.doc().unwrap().doc;
+            let runs: usize = ids.iter().filter_map(|id| text_of(d, NodeId(*id))).map(|t| t.runs.len()).sum();
+            println!("{runs} runs in 8 frames: typing {typing:.1} ms, a move below them {edit:.3} ms, a drag frame {frame:.3} ms (medians)");
+        }
+    }
+
     #[test]
     fn release_and_remove() {
         let (mut s, a, b) = setup();

docs/target-app-parity.md

@@ -468,6 +468,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
     Nearly unchanged appearance is expected when reducing points. Percentage/auto-simplify controls, the corner-angle UI and Show Original are not implemented; the checklist is partial, without raising scores.
     Simplify and Add Anchor Points record no undo step when they change no path, and a path they leave as it is keeps its live shape.
 - **Type:** Character size, leading, baseline shift and horizontal-scale controls account for object/group transforms (#1034): exact document-point sizes, mixed-value display and keyboard increments, range edits, resettable horizontal stretch, undo/redo and native save/reopen (`tests_typescale`, Character panel tests). Point size is the transformed em-height axis, independent of glyph ink bounds; affine rotation, reflection and shear are retained. Group scaling still scales text; preserving font sizes is a separate enhancement. Behaviour reference: [Transform text](https://helpx.adobe.com/illustrator/desktop/design-with-text/edit-format-text/transform-text.html) (public documentation). Text Wrap, Type on a Path effects (Rainbow/Skew/3D Ribbon/Stair Step/Gravity) and Options (Flip, Align to Path, Spacing; OK keeps the type where it is), type on a path's start, centre and end brackets (Selection and Direct Selection drag them to set where the type runs, slide it along its path and, across the path, flip it: #429), Character and Paragraph Styles (override-preserving redefine), paragraph attributes per paragraph (alignment, indents, spacing, hyphenation, hanging punctuation, tab stops, paragraph style, direction, mojikumi and leading model; the Paragraph, Tabs and Paragraph Styles panels act on the paragraphs the Type tool's selection or caret touches, Return continues the paragraph's style, threads and SVG keep them), Area Type Options (rows/columns/inset/first baseline, vertical Align Top/Center/Bottom/Justify per row/column, reflowing non-rectangular frames and text wrap), threaded text across any closed shapes, Fit Headline, the Every-line Composer for justified and ragged text (paragraph-wide breaks for an even rag, Single-line per object from the Paragraph panel menu or `text.setFormat {composer}`). Area type resizes like Illustrator's type areas: a bounding-box handle drag (Selection tool) or Area Type Options Width/Height size the frame and the text reflows at its size (threads too); Direct Selection drags a frame corner or edge (`text.reshapeArea`). Point type still scales. Auto Size: the frame height follows the text after every edit, in the same undo step (rectangular frames of horizontal type in one row; columns get the least height that holds them; a height set by hand or threading turns it off), and the "Auto Size New Area Type" preference gives it to new area type. Shrink Text to Fit (beyond Illustrator): overflowing area type scales its sizes, leading and baseline shifts down by the largest factor (in 0.1 pt steps of the first run, down to a minimum percentage) that fits, at layout time so every export sees it. `text.areaOptions` and `document.inspect` report overflow and the shrink factor. The Selection tool shows the type widget beside selected type's bounding box (hollow on point type, filled on area type); double-clicking it converts between them (Type › Convert To Area Type / Convert To Point Type), keeping the text and its styles in place. Variable fonts list every named instance as a style of their family (Figtree Light to Black and their italics, Bahnschrift, Segoe UI Variable), and the instance chosen is shaped, drawn, outlined and measured at its axis settings; PDF and SVG embed it as that instance. Variation axis sliders are not done. Beyond Illustrator: inline graphics (`text.insertInline`, Type ▸ Insert Inline Symbol) set a document symbol in a run of type as one character that flows, wraps and justifies like a glyph (InDesign-style inline objects, e.g. mana symbols in card rules text): sized to a multiple of the font size, centred on the cap height, opening up tall lines, following Redefine Symbol; a character of its run's size for Character Alignment and Top-to-Top leading, placed by the bidi reordering in right-to-left text (a neutral U+FFFC); drawn on the canvas, kept as live text plus a `<use>` in SVG and as vector art in PDF; Backspace removes it whole. Hebrew and Arabic are laid out right to left with the Unicode bidirectional algorithm (embedded Latin and numbers keep their direction, lines put in visual order after wrapping) in point, area and path type, with caret, selection and arrow keys in visual order; each paragraph's direction comes from its first strong character unless Paragraph Direction sets it (Paragraph panel with Show Indic Options, `text.setFormat {direction}`), and new type aligns to the start of its direction. SVG export outlines right-to-left text. Arabic typefaces: builds made with a craft-fonts checkout that holds its Arabic families (Noto Sans Arabic, the Arabic fallback, plus Cairo, Tajawal, Almarai, IBM Plex Sans Arabic, Noto Kufi Arabic, Amiri, Alexandria, Fustat, Reem Kufi, Aref Ruqaa and Gulzar once craft-fonts adds them, storytold/craft-fonts#11; releases get them when their craft-fonts pin moves) list them in the font menus on desktop; the web build fetches them from beside the wasm (Noto Sans Arabic before start, so Arabic never shows as boLine truncated
+  An edit copies a thread's story only when it changes one of the thread's frames: with a story of 50,000 characters threaded through 8 frames with a style change at every word, moving another object took 7.8 ms and now takes 0.013 ms, and a frame of a drag took 7.2 ms and now takes 0.012 ms (release build, `threaded_story_edit_timing`).
 - **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 and Brazilian Portuguese every menu label, Brazilian Portuguese the shell's `tl!` literals too (tested); German, Spanish, French, Italian, Japanese, Russian and Ukrainian 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).
   Disable, Enable, Unlink and Link Opacity Mask and Opacity Mask Options (`transparency.setOpacityMask`) record no undo step when every mask they act on already has the flags they set.