Fix #569: Shape Builder merges leave no stray edges in their outlines

Merging regions united the faces' outlines with a boolean union. Each
face had been refit on its own, so two faces' shared edge could come out
a hair apart: the union then kept the edge between them, or a spur of
it, and a stroked merged shape (unfilled art) showed a stray line inside.
Subtracting the merged shape from the others left slivers along it too.

pathops::FaceMerger merges faces from the planar arrangement itself,
made once per merge: their contours share their edges exactly, so the
edges between merged faces vanish. The Shape Builder builds the merged
shape that way, and what is left of each shape from its faces not merged
(rather than subtracting the merged shape from it). Selections with open
paths keep the old union.

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

ROADMAP.md

@@ -4,7 +4,7 @@ VectorCraft is a clean-room, open-source, pure-Rust reimplementation of the Adob
 
 This file tracks **how far we are and what's left**. Time estimates are wall-clock hours of continuous Claude Opus 5.5 agent work (including builds and the CI gate), given both for **one agent** and for **4–6 parallel agents** on disjoint crates. They are counted from the remaining work (see [Parity estimate](#parity-estimate)), calibrated against measured throughput, and updated as work lands.
 
-_Last updated: 2026-10-09 (after placed documents (File › Place of a `.vectorcraft` file, linked), M4.14–M4.98, M8.1–M8.20, M14.4–M14.7, the MCP prompts, resource templates, completions and logging, the 2026-10-07 issue fixes, the Layers panel rework and the 2026-10-08 community contributions: Pen and shape-tool modifiers, path editing, Layers, Artboards, Swatches/Brushes and dialogs, Japanese composition and PDF text, Hebrew/Arabic type, interface languages (Spanish among them), the font menu, saved selections, MCP and EPS, the missing-font export warning (#319), open paths in the Shape Builder (#374), PDF/`.ai` import keeping strokes live and hidden and nested layers (#372), per-corner Live Corners (#381), SVG type opening at the size it draws at (#396), the Wayland clipboard with copied-file paste (#398), ruler guides that can be selected, moved and deleted (#414), two batches of Preferences options that were stored but unused (#394), skeleton reads with `document.find` for large documents over MCP (#416) and anchors dragged with Direct Selection snapping to points, segments and smart guides, clipped SVG layers opening as clipping layers (#420), PDF/`.ai` layers read on pages of tens of millions of operators (#419), Document Color Mode putting RGB greys on the black plate on request and converting pattern tiles (#421) and movable dialogs with Offset Path's Joins dropdown (#423) and numeric fields scrubbed by dragging their label (#400), stroke weight presets in the stroke unit (#426), the Appearance panel's Opacity popup (#430), type on a path brackets, flip and Type on a Path Options that keep the type in place (#429), Layers rows dragged with a ghost and dropped on the trash or New Layer (#435), tool flyouts opening on a right or corner press (#436), tool flyouts torn off into floating toolbars (#437), the Shape Builder's erase mode deleting edges too (#439), installed fonts registered outside the font folders and fonts installed while the app runs listed in the font menus (#443), alike circular corners on rectangles that aren't square (#442), tool cursors as OS cursors so the pointer no longer trails over the canvas (#444), artboards copied and pasted with their art (#446), guides dragged out of the rulers snapping to the art's side midpoints and the artboards, artboard guides (#451), the toolbar's double arrow and Window › Toolbars › Double Column switching between one and two columns in any window height (#455), the Smart Guides display options and Snapping Tolerance (#394), Font Size and Leading combo boxes with preset lists in the Character and Properties panels and the Control bar (#458), screenshots pasted from the clipboard on macOS, Windows and the web (#457), the Properties panel's Fill and Stroke matching the Control bar's, Illustrator's Window menu panel keys (Shift+F7 for Align…) and Alt-wheel zoom whatever the wheel preference (Discord feedback), the point/area type widget (Discord feedback), groups in legacy Illustrator .ai files (Discord feedback), a rotating desktop log file (#465), image controls in the Control bar and Properties panel: Image Trace with its presets, Mask, Crop Image, Embed/Unembed, and an Image Trace object's preset, Expand and Release (Discord feedback), the Pen working as the Anchor Point tool with Alt held over a selected path's handles and anchors, and anchor buttons that convert, connect and cut paths at the selected anchors (Discord feedback), Japanese hanging punctuation (burasagari, #463), Escape leaving Presentation Mode with no screen mode saved across restarts, documents dropped on the desktop app opening as tabs of their own and a note that an `.ai` file's art off its artboards isn't in the part read (#472), Add/Delete Anchor Point tool icons drawn from the Pen tool's (#479), EPS import running more of PostScript Level 3 (patterns, Type 3 fonts, mesh shadings, masked images, VM and resource probes), and a preview that stands in saying why (Discord feedback, #466, #469, #474), chords with modifiers recorded in Edit › Keyboard Shortcuts (#487), Type Object Selection by Path Only (#394), Anti-aliased Artwork (#394), Show Pixel Grid (#394), pen and touch taps pressed and lifted within one frame clicking where they tapped and the Linux/Wayland pen-tablet workaround (#491), bounding-box resizes landing on the other objects' anchors and centres with labels, and the Rotate, Scale, Shear and Reflect tools snapping their drags (#482), Shift-drag marquees that toggle the objects or anchors they reach and a Lasso whose Alt subtraction keeps the rest of the selection (#483), the mouse wheel stepping the focused numeric field, the dialogs' and preferences' drag values made such fields (#485), Show The Home Screen When No Documents Are Open (#394), Numbers Without Units Are Points (#394), Auto-Collapse Iconic Panels (#394), the Contextual Task Bar moved by its handle and its More Options menu pinning and resetting its position, a designed macOS DMG windoLine truncated
+_Last updated: 2026-10-09 (after placed documents (File › Place of a `.vectorcraft` file, linked), M4.14–M4.98, M8.1–M8.20, M14.4–M14.7, the MCP prompts, resource templates, completions and logging, the 2026-10-07 issue fixes, the Layers panel rework and the 2026-10-08 community contributions: Pen and shape-tool modifiers, path editing, Layers, Artboards, Swatches/Brushes and dialogs, Japanese composition and PDF text, Hebrew/Arabic type, interface languages (Spanish among them), the font menu, saved selections, MCP and EPS, the missing-font export warning (#319), open paths in the Shape Builder (#374), PDF/`.ai` import keeping strokes live and hidden and nested layers (#372), per-corner Live Corners (#381), SVG type opening at the size it draws at (#396), the Wayland clipboard with copied-file paste (#398), ruler guides that can be selected, moved and deleted (#414), two batches of Preferences options that were stored but unused (#394), skeleton reads with `document.find` for large documents over MCP (#416) and anchors dragged with Direct Selection snapping to points, segments and smart guides, clipped SVG layers opening as clipping layers (#420), PDF/`.ai` layers read on pages of tens of millions of operators (#419), Document Color Mode putting RGB greys on the black plate on request and converting pattern tiles (#421) and movable dialogs with Offset Path's Joins dropdown (#423) and numeric fields scrubbed by dragging their label (#400), stroke weight presets in the stroke unit (#426), the Appearance panel's Opacity popup (#430), type on a path brackets, flip and Type on a Path Options that keep the type in place (#429), Layers rows dragged with a ghost and dropped on the trash or New Layer (#435), tool flyouts opening on a right or corner press (#436), tool flyouts torn off into floating toolbars (#437), the Shape Builder's erase mode deleting edges too (#439), installed fonts registered outside the font folders and fonts installed while the app runs listed in the font menus (#443), alike circular corners on rectangles that aren't square (#442), tool cursors as OS cursors so the pointer no longer trails over the canvas (#444), artboards copied and pasted with their art (#446), guides dragged out of the rulers snapping to the art's side midpoints and the artboards, artboard guides (#451), the toolbar's double arrow and Window › Toolbars › Double Column switching between one and two columns in any window height (#455), the Smart Guides display options and Snapping Tolerance (#394), Font Size and Leading combo boxes with preset lists in the Character and Properties panels and the Control bar (#458), screenshots pasted from the clipboard on macOS, Windows and the web (#457), the Properties panel's Fill and Stroke matching the Control bar's, Illustrator's Window menu panel keys (Shift+F7 for Align…) and Alt-wheel zoom whatever the wheel preference (Discord feedback), the point/area type widget (Discord feedback), groups in legacy Illustrator .ai files (Discord feedback), a rotating desktop log file (#465), image controls in the Control bar and Properties panel: Image Trace with its presets, Mask, Crop Image, Embed/Unembed, and an Image Trace object's preset, Expand and Release (Discord feedback), the Pen working as the Anchor Point tool with Alt held over a selected path's handles and anchors, and anchor buttons that convert, connect and cut paths at the selected anchors (Discord feedback), Japanese hanging punctuation (burasagari, #463), Escape leaving Presentation Mode with no screen mode saved across restarts, documents dropped on the desktop app opening as tabs of their own and a note that an `.ai` file's art off its artboards isn't in the part read (#472), Add/Delete Anchor Point tool icons drawn from the Pen tool's (#479), EPS import running more of PostScript Level 3 (patterns, Type 3 fonts, mesh shadings, masked images, VM and resource probes), and a preview that stands in saying why (Discord feedback, #466, #469, #474), chords with modifiers recorded in Edit › Keyboard Shortcuts (#487), Type Object Selection by Path Only (#394), Anti-aliased Artwork (#394), Show Pixel Grid (#394), pen and touch taps pressed and lifted within one frame clicking where they tapped and the Linux/Wayland pen-tablet workaround (#491), bounding-box resizes landing on the other objects' anchors and centres with labels, and the Rotate, Scale, Shear and Reflect tools snapping their drags (#482), Shift-drag marquees that toggle the objects or anchors they reach and a Lasso whose Alt subtraction keeps the rest of the selection (#483), the mouse wheel stepping the focused numeric field, the dialogs' and preferences' drag values made such fields (#485), Show The Home Screen When No Documents Are Open (#394), Numbers Without Units Are Points (#394), Auto-Collapse Iconic Panels (#394), the Contextual Task Bar moved by its handle and its More Options menu pinning and resetting its position, a designed macOS DMG windoLine truncated
 
 ## Where we are
 

crates/engine/src/cmd/buildcmds.rs

@@ -250,7 +250,8 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
         // Edges come back from the planar map within a few hundredths of the outlines they follow.
         let cut_tol = step * 0.2;
         let picked: Vec<&po::Region> = touched.regions.iter().filter_map(|&i| map.regions.get(i).map(|r| &r.0)).collect();
-        let union = if picked.is_empty() { PathData::default() } else { po::merge_regions(&picked) };
+        let faces = po::FaceMerger::new(&map.shapes);
+        let union = if picked.is_empty() { PathData::default() } else { faces.merge(&picked) };
         // The merged shape takes the look of the front-most object over the first region, or (for an
         // area only lines enclose) of the front-most object and the current fill.
         let covered = picked.iter().find_map(|r| r.top());
@@ -278,7 +279,17 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
                 // its outline, shared edges from every shape along them).
                 let in_union = picked.iter().any(|r| r.sources.contains(&i));
                 let mut rest = if in_union {
-                    po::boolean(&sh.path, sh.rule, &union, FillRule::NonZero, po::BoolOp::Difference)
+                    // What is left of it: its faces not merged or deleted, united. They share their
+                    // edges with the merged faces exactly, where subtracting the merged shape left
+                    // slivers of the outline along them (#569).
+                    let kept: Vec<&po::Region> = map
+                        .regions
+                        .iter()
+                        .enumerate()
+                        .filter(|(k, (r, ..))| r.sources.contains(&i) && !touched.regions.contains(k))
+                        .map(|(_, (r, ..))| r)
+                        .collect();
+                    if kept.is_empty() { PathData::default() } else { faces.merge(&kept) }
                 } else {
                     b::node_outline(leaf).map_or_else(|| sh.path.clone(), |o| o.0)
                 };

crates/engine/src/tests_build.rs

@@ -704,3 +704,67 @@ fn image_trace_object_remembers_its_settings() {
     let id = st.selection.in_paint_order(&st.doc)[0];
     assert!(st.doc.node(id).unwrap().trace.is_none());
 }
+
+/// #569: merging the regions of a rosette of unfilled ellipses (each off the centre, petals
+/// overlapping) merges every region the drags touch into one shape whose outline is only the
+/// merged area's border: no edge between merged regions stays in, as a stray line or a spur.
+#[test]
+fn shape_builder_merges_rosette_rings_cleanly() {
+    let mut s = session();
+    s.execute("paint.setFill", &json!({"none": true})).unwrap();
+    let c = [400.0, 320.0];
+    let mut petals = vec![];
+    for k in 0..8 {
+        let id =
+            s.execute("shape.ellipse", &json!({"x": c[0] - 60.0, "y": c[1] - 172.0, "width": 120, "height": 324})).unwrap()["id"].as_u64().unwrap();
+        s.execute("select.set", &json!({"ids": [id]})).unwrap();
+        s.execute("object.rotate", &json!({"angle": 45.0 * f64::from(k), "origin": c})).unwrap();
+        petals.push(id);
+    }
+    s.execute("select.set", &json!({"ids": petals})).unwrap();
+    // Drags around the ring of petals, from its inside out; the outer ones miss the petals.
+    let mut merged = None;
+    for radius in [110.0, 150.0, 190.0, 230.0] {
+        let pts: Vec<[f64; 2]> = (0..=120)
+            .map(|i| {
+                let a = std::f64::consts::TAU * f64::from(i) / 120.0;
+                [c[0] + radius * a.cos(), c[1] + radius * a.sin()]
+            })
+            .collect();
+        if let Ok(r) = s.execute("shapeBuilder.merge", &json!({"points": pts})) {
+            merged = r["merged"].as_u64().map(NodeId);
+        }
+    }
+    let (ring, rule) = outline(&node(&s, merged.expect("the drags merged")));
+    let inside = |p: kurbo::Point| kurbo::Shape::winding(&ring.to_bezpath(), p) != 0;
+    for k in 0..8 {
+        let a = std::f64::consts::FRAC_PI_4 * f64::from(k) - std::f64::consts::FRAC_PI_2;
+        let tip = kurbo::Point::new(c[0] + 165.0 * a.cos(), c[1] + 165.0 * a.sin());
+        assert!(inside(tip), "petal {k}'s tip at {tip:?} is in the merged shape");
+    }
+    // What is left of the petals stays out of the merged area (a piece left over it is a stray
+    // outline inside it).
+    let merged = merged.unwrap();
+    for id in s.doc().unwrap().selection.objects.clone().into_iter().filter(|id| *id != merged) {
+        let (p, r) = outline(&node(&s, id));
+        let over = vectorcraft_pathops::boolean(&p, r, &ring, rule, vectorcraft_pathops::BoolOp::Intersect);
+        let a = vectorcraft_pathops::area(&over, FillRule::NonZero);
+        assert!(a < 1.0, "{id:?} overlaps the merged shape by {a} pt²");
+    }
+    // No outline is a sliver or carries a spur (2 × area / length: its mean width).
+    for id in s.doc().unwrap().selection.objects.clone() {
+        let (p, r) = outline(&node(&s, id));
+        for sp in &p.subpaths {
+            let one = PathData::new(vec![sp.clone()]);
+            let width = 2.0 * vectorcraft_pathops::area(&one, r) / one.length();
+            assert!(width > 1.0, "{id:?} has an outline {width:.3} pt wide on average, {:.1} pt long", one.length());
+        }
+    }
+    // The ring is one outline around the centre (the regions the drags missed aside): the
+    // petals' tips are no outlines of their own beside it, apart by a hairline.
+    let big = ring.subpaths.iter().filter(|sp| vectorcraft_pathops::area(&PathData::new(vec![(*sp).clone()]), rule) > 100.0).count();
+    assert_eq!(big, 2, "{:?}", ring.subpaths.iter().map(|sp| vectorcraft_pathops::area(&PathData::new(vec![sp.clone()]), rule)).collect::<Vec<_>>());
+    // An inner edge would be a second outline over the merged area: normalising would drop it.
+    let clean = vectorcraft_pathops::normalize(&ring, rule);
+    assert!((ring.length() - clean.length()).abs() < 1.0, "{} pt of outline, {} without inner edges", ring.length(), clean.length());
+}

crates/pathops/src/lib.rs

@@ -19,7 +19,7 @@ pub use edit::{
     split_into_grid,
 };
 pub use offset::{Cap, Join, offset_path, outline_stroke, stroke_region};
-pub use pathfinder::{PathfinderOp, Region, Shape, merge_regions, pathfinder, region_at, regions};
+pub use pathfinder::{FaceMerger, PathfinderOp, Region, Shape, merge_regions, pathfinder, region_at, regions};
 pub use planar::{BuilderArrangement, SHAPE_BUILDER_MAX_SEGMENTS, cut_out, encloses_area, interior_point, live_paint, shape_builder};
 
 /// Errors from fallible operations.

crates/pathops/src/pathfinder.rs

@@ -7,7 +7,7 @@ use kurbo::{BezPath, ParamCurve, ParamCurveNearest, Point, Shape as _};
 use linesweeper::topology::ContourIdx;
 use vectorcraft_geom::{FillRule, PathData};
 
-use crate::boolean::{Arrangement, Multi, Seg, all_contours_to_path, contours_to_path, fill_bezpath, segs_to_subpath, unite_all};
+use crate::boolean::{Arrangement, Multi, Seg, all_contours_to_path, contours_to_path, fill_bezpath, normalize_bez, segs_to_subpath, unite_all};
 
 /// A filled shape in a Pathfinder stack. `key` identifies its paint (e.g. a hashed fill colour);
 /// results carry the key of the object whose paint they keep.
@@ -264,6 +264,63 @@ pub fn merge_regions(regions: &[&Region]) -> PathData {
     unite_all(&v)
 }
 
+/// Merges faces of the planar arrangement of a set of shapes (as [`regions`] gives them) into one
+/// path, from the arrangement itself, made once for every merge of the set: the faces' contours
+/// share their edges exactly, so the edges between merged faces vanish. Uniting the faces' tidied
+/// outlines ([`merge_regions`]) could leave those edges in, as a hairline gap or a spur, where
+/// neighbouring faces' outlines had been refit apart. Shapes with open paths (faces lines cut)
+/// fall back to [`merge_regions`].
+pub struct FaceMerger {
+    /// The arrangement (`None`: open paths, or the sweep failed) and the number of shapes.
+    arr: Option<Arrangement>,
+    shapes: usize,
+}
+
+impl FaceMerger {
+    pub fn new(shapes: &[Shape]) -> Self {
+        let has_open = shapes.iter().flat_map(|s| &s.path.subpaths).any(|sp| !sp.closed && sp.anchors.len() >= 2);
+        Self { arr: if has_open { None } else { arrangement(shapes) }, shapes: shapes.len() }
+    }
+
+    /// The union of `faces`.
+    pub fn merge(&self, faces: &[&Region]) -> PathData {
+        self.exact(faces).filter(|p| !p.is_empty()).unwrap_or_else(|| merge_regions(faces))
+    }
+
+    fn exact(&self, faces: &[&Region]) -> Option<PathData> {
+        let arr = self.arr.as_ref()?;
+        // Each face by its coverage and a point inside it.
+        let mut picks: Vec<(Vec<bool>, Point)> = Vec::with_capacity(faces.len());
+        for r in faces {
+            let mut mask = vec![false; self.shapes];
+            for &s in &r.sources {
+                *mask.get_mut(s)? = true;
+            }
+            picks.push((mask, crate::planar::interior_point(&r.path)?));
+        }
+        let mut raw = BezPath::new();
+        let mut done: Vec<&[bool]> = vec![];
+        for (mask, _) in &picks {
+            if done.contains(&mask.as_slice()) {
+                continue;
+            }
+            done.push(mask);
+            let c = arr.contours(|k| k == mask.as_slice());
+            for group in c.grouped() {
+                let mut face = BezPath::new();
+                for i in group {
+                    face.extend(c[i].path.iter());
+                }
+                if picks.iter().any(|(m, p)| m == mask && face.winding(*p) != 0) {
+                    raw.extend(face.iter());
+                }
+            }
+        }
+        let merged = normalize_bez(&raw, FillRule::NonZero).ok()?;
+        Some(all_contours_to_path(&merged, &arr.tidy))
+    }
+}
+
 /// Pathfinder Outline: split every shape's boundary at junctions of the arrangement.
 fn outline_edges(arr: &Arrangement, shapes: &[Shape]) -> Vec<Shape> {
     let junctions = arr.junctions();