Layers: counting a row's art for its first thumbnail stops after 20,000 steps

A row drawn before was counted with no limit on its objects, so symbols
that each hold many instances of the next made the count grow with the
product of the instances, and the Layers panel could stall on such a file.

- Counting stops after 20,000 steps: each object, subpath, text run and
  symbol looked at is one step, and every 64 bytes of a symbol or placed
  file name looked up is one more. A row that needs more gets its
  thumbnail on the thumbnail thread, and the painted thumbnail draws
  nothing for it.
- A row drawn before counts at most 20,000 objects.

crates/ui-egui/src/panels/layers/thumbs.rs

@@ -95,6 +95,12 @@ const IMAGE_PIXELS_PER_ANCHOR: usize = 100;
 /// worker.
 const MAX_THUMB_NESTING: usize = 8;
 
+/// The most steps counting a node takes: each object, subpath, text run and symbol looked at is
+/// one step, and so is every 64 bytes of a symbol or placed file name looked up. A node that
+/// needs more has its thumbnail made on the worker, and the painted thumbnail draws nothing for
+/// it. A row drawn before counts at most this many objects too.
+const THUMB_COUNT_STEPS: usize = 20_000;
+
 /// At most this long (ms) and this many thumbnails are rendered on the UI thread in a pass; the
 /// rest go to the worker, or wait for the next pass where there is none.
 const THUMB_BUDGET_MS: f64 = 8.0;
@@ -113,12 +119,19 @@ pub(crate) fn thumb_options() -> vectorcraft_render::RenderOptions {
 /// glyphs, mesh points and image pixels count as anchors too ([`LIGHT_THUMB_ANCHORS`]), and a
 /// symbol instance and a placed document count the objects and anchors of the art they show: the
 /// symbol's art in `doc`, and the placed file's art when it has been read (a placed document whose
-/// file has not been read, or art nested deeper than [`MAX_THUMB_NESTING`], is not small).
+/// file has not been read, or art nested deeper than [`MAX_THUMB_NESTING`], is not small). A node
+/// that takes more than [`THUMB_COUNT_STEPS`] steps to count is not small.
 fn small_node(n: &Node, nodes: usize, anchors: usize, masks: bool, doc: Option<&Document>) -> bool {
+    weigh(n, nodes, anchors, masks, doc).0
+}
+
+/// [`small_node`], and the steps counting took.
+fn weigh(n: &Node, nodes: usize, anchors: usize, masks: bool, doc: Option<&Document>) -> (bool, usize) {
     /// What may still be counted.
     struct Left {
         nodes: usize,
         anchors: usize,
+        steps: usize,
         masks: bool,
     }
     impl Left {
@@ -132,6 +145,20 @@ fn small_node(n: &Node, nodes: usize, anchors: usize, masks: bool, doc: Option<&
                 None => false,
             }
         }
+        /// Take `steps` more: false past the limit.
+        fn step(&mut self, steps: usize) -> bool {
+            match self.steps.checked_sub(steps) {
+                Some(left) => {
+                    self.steps = left;
+                    true
+                }
+                None => false,
+            }
+        }
+        /// Look up `name`: false past the step limit.
+        fn look_up(&mut self, name: &str) -> bool {
+            self.step(1 + name.len() / 64)
+        }
         /// Count one more object: false past the limit.
         fn object(&mut self) -> bool {
             match self.nodes.checked_sub(1) {
@@ -142,14 +169,9 @@ fn small_node(n: &Node, nodes: usize, anchors: usize, masks: bool, doc: Option<&
                 None => false,
             }
         }
-    }
-    fn own_anchors(n: &Node) -> usize {
-        match &n.kind {
-            NodeKind::Text(t) => match &t.kind {
-                TextKind::Area { frame: p } | TextKind::OnPath { path: p, .. } => p.anchor_count(),
-                TextKind::Point => 0,
-            },
-            _ => n.path_data().map_or(0, PathData::anchor_count),
+        /// Count the anchors of `p`, a step for each subpath.
+        fn path(&mut self, p: &PathData) -> bool {
+            p.subpaths.iter().all(|s| self.step(1) && self.take(s.anchors.len()))
         }
     }
     /// Count `art`, drawn in place of an object (`doc` its document), one level deeper.
@@ -158,11 +180,26 @@ fn small_node(n: &Node, nodes: usize, anchors: usize, masks: bool, doc: Option<&
     }
     /// Count symbol `name` of `doc` (one that doesn't exist draws nothing).
     fn symbol(name: &str, doc: &Document, left: &mut Left, depth: usize) -> bool {
-        doc.symbols.iter().find(|s| s.name == name).is_none_or(|s| inside(&s.art, doc, left, depth))
+        for s in &doc.symbols {
+            if !left.look_up(name) {
+                return false;
+            }
+            if s.name == name {
+                return inside(&s.art, doc, left, depth);
+            }
+        }
+        true
     }
     /// Count what `n` draws besides its children.
     fn own(n: &Node, doc: Option<&Document>, left: &mut Left, depth: usize) -> bool {
-        if !left.take(own_anchors(n)) {
+        let path = match &n.kind {
+            NodeKind::Text(t) => match &t.kind {
+                TextKind::Area { frame: p } | TextKind::OnPath { path: p, .. } => Some(p),
+                TextKind::Point => None,
+            },
+            _ => n.path_data(),
+        };
+        if !path.is_none_or(|p| left.path(p)) {
             return false;
         }
         let Some(doc) = doc else { return true };
@@ -170,23 +207,38 @@ fn small_node(n: &Node, nodes: usize, anchors: usize, masks: bool, doc: Option<&
             // Glyphs are counted only up to one past what the anchors left allow.
             NodeKind::Text(t) => t.runs.iter().all(|r| {
                 let glyphs = r.text.chars().take(left.anchors / GLYPH_ANCHORS + 1).count();
-                left.take(glyphs.saturating_mul(GLYPH_ANCHORS)) && r.inline.as_ref().is_none_or(|a| symbol(&a.symbol, doc, left, depth))
+                left.step(1)
+                    && left.take(glyphs.saturating_mul(GLYPH_ANCHORS))
+                    && r.inline.as_ref().is_none_or(|a| symbol(&a.symbol, doc, left, depth))
             }),
             NodeKind::SymbolInstance { symbol: name, .. } => symbol(name, doc, left, depth),
             NodeKind::Mesh(m) => left.take(m.points.len().saturating_mul(MESH_POINT_ANCHORS)),
             NodeKind::Image(im) => left.take((im.width as usize).saturating_mul(im.height as usize) / IMAGE_PIXELS_PER_ANCHOR),
-            NodeKind::PlacedDocument(p) => vectorcraft_doc::placed_document::read_cached(p).is_some_and(|e| inside(&e.art, &e.doc, left, depth)),
+            NodeKind::PlacedDocument(p) => {
+                left.look_up(&p.key) && vectorcraft_doc::placed_document::read_cached(p).is_some_and(|e| inside(&e.art, &e.doc, left, depth))
+            }
             _ => true,
         }
     }
     fn count(n: &Node, doc: Option<&Document>, left: &mut Left, depth: usize) -> bool {
-        if !own(n, doc, left, depth) {
+        if !(left.step(1) && own(n, doc, left, depth)) {
             return false;
         }
         let mask = n.extras().mask.as_deref().filter(|m| left.masks && !m.disabled).map(|m| &m.art);
         n.children().into_iter().flatten().chain(mask).all(|c| left.object() && count(c, doc, left, depth))
     }
-    count(n, doc, &mut Left { nodes, anchors, masks }, 0)
+    let mut left = Left { nodes, anchors, steps: THUMB_COUNT_STEPS, masks };
+    let small = count(n, doc, &mut left, 0);
+    (small, THUMB_COUNT_STEPS - left.steps)
+}
+
+/// Is row `n` (an object of `doc`) light enough to make its thumbnail on the UI thread? `quick`:
+/// whether its last thumbnail was quick to render, if it has one.
+fn light_row(n: &Node, quick: Option<bool>, doc: &Document) -> bool {
+    match quick {
+        Some(quick) => quick && small_node(n, THUMB_COUNT_STEPS, LIGHT_THUMB_ANCHORS, true, Some(doc)),
+        None => small_node(n, LIGHT_THUMB_NODES, LIGHT_THUMB_ANCHORS, true, Some(doc)),
+    }
 }
 
 /// The thumbnail of `n` (`px` pixels square) as `r` renders it, on the UI thread or the worker.
@@ -275,11 +327,7 @@ impl Thumbs {
         let in_budget = self.spent.1 < THUMB_BUDGET_MS && self.spent.2 < THUMB_BUDGET_COUNT;
         // A row drawn before is light when its last thumbnail was quick to render, unless its
         // object now has more anchors than the limit.
-        let doc = Some(src.doc.as_ref());
-        let light = match light {
-            Some(quick) => quick && small_node(n, usize::MAX, LIGHT_THUMB_ANCHORS, true, doc),
-            None => small_node(n, LIGHT_THUMB_NODES, LIGHT_THUMB_ANCHORS, true, doc),
-        } && in_budget;
+        let light = light_row(n, light, src.doc) && in_budget;
         if !light && let Some(w) = &self.worker {
             let (doc, node) = (src.doc.clone(), n.clone());
             let job: ThumbJob = (key, Arc::downgrade(n), px);
@@ -327,7 +375,8 @@ pub(crate) fn real_thumb(ui: &Ui, src: &ThumbSource, n: &Arc<Node>, r: egui::Rec
 }
 
 /// Tiny vector thumbnail painted with egui (fallback when rendering isn't possible). A node with
-/// more than [`LIGHT_THUMB_ANCHORS`] anchors paints nothing, since this runs on every frame and
+/// more than [`LIGHT_THUMB_ANCHORS`] anchors, or that takes more than [`THUMB_COUNT_STEPS`] steps
+/// to count, paints nothing, since this runs on every frame and
 /// measuring its bounds takes time in proportion to its anchor count.
 pub(super) fn thumb(ui: &Ui, n: &Node, r: egui::Rect) {
     if !small_node(n, usize::MAX, LIGHT_THUMB_ANCHORS, false, None) {
@@ -563,6 +612,61 @@ mod tests {
         assert!(!small_in(&doc, &instance(1, "Deeper")), "one level deeper");
     }
 
+    /// A document of symbols `L0` to `L{levels}`: `L0` is an empty group, and each one above it a
+    /// group of `fan` instances of the one below.
+    fn fan_out(levels: usize, fan: usize) -> Document {
+        let mut doc = with_symbol("L0", Node::group(NodeId(10), vec![]));
+        let mut id = 100;
+        for level in 1..=levels {
+            let below = format!("L{}", level - 1);
+            let kids = (0..fan)
+                .map(|_| {
+                    id += 1;
+                    Arc::new(instance(id, &below))
+                })
+                .collect();
+            id += 1;
+            doc.symbols.push(vectorcraft_doc::Symbol { name: format!("L{level}"), art: Arc::new(Node::group(NodeId(id), kids)), dynamic: false });
+        }
+        doc
+    }
+
+    #[test]
+    fn symbols_that_each_hold_many_instances_are_counted_within_the_step_limit() {
+        // 40 instances on each of 4 levels: 40⁴ (2,560,000) empty groups, and no anchors.
+        let doc = fan_out(4, 40);
+        let top = instance(1, "L4");
+        assert!(!light_row(&top, Some(true), &doc), "a row drawn before, quick to render");
+        assert!(!light_row(&top, None, &doc), "a row not drawn yet");
+        let (small, steps) = weigh(&top, THUMB_COUNT_STEPS, usize::MAX, true, Some(&doc));
+        assert!(!small && steps <= THUMB_COUNT_STEPS, "{steps} steps");
+        let (small, steps) = weigh(&top, usize::MAX, usize::MAX, true, Some(&doc));
+        assert!(!small && steps <= THUMB_COUNT_STEPS, "{steps} steps without a limit on objects");
+        // 5 instances on each of 3 levels stay light once drawn quickly.
+        let doc = fan_out(3, 5);
+        assert!(light_row(&instance(1, "L3"), Some(true), &doc));
+        assert!(!light_row(&instance(1, "L3"), Some(false), &doc), "drawn slowly");
+    }
+
+    #[test]
+    fn empty_subpaths_text_runs_and_symbol_lookups_take_steps() {
+        let doc = Document::new(200.0, 200.0);
+        let style = Appearance::basic(Paint::None, Paint::solid(Color::BLACK), 1.0);
+        let empty = Node::path(NodeId(1), PathData::new(vec![SubPath::default(); THUMB_COUNT_STEPS]), style.clone());
+        assert!(!small_in(&doc, &empty), "a path of many empty subpaths");
+        let fewer = Node::path(NodeId(1), PathData::new(vec![SubPath::default(); 100]), style);
+        assert!(small_in(&doc, &fewer), "a path of a few");
+        let mut t = TextObject::point(Point::ZERO, "", CharStyle::default());
+        t.runs = vec![vectorcraft_doc::TextRun::new("", CharStyle::default()); THUMB_COUNT_STEPS];
+        assert!(!small_in(&doc, &Node::new(NodeId(2), NodeKind::Text(Box::new(t)))), "type of many empty runs");
+        let mut doc = with_symbol("S", rect(10));
+        for i in 0..THUMB_COUNT_STEPS {
+            doc.symbols.push(vectorcraft_doc::Symbol { name: format!("Other {i}"), art: Arc::new(rect(11)), dynamic: false });
+        }
+        assert!(small_in(&doc, &instance(1, "S")), "the first of many symbols");
+        assert!(!small_in(&doc, &instance(1, &format!("Other {}", THUMB_COUNT_STEPS - 1))), "the last of many symbols");
+    }
+
     #[test]
     fn the_painted_thumbnail_leaves_out_a_long_path() {
         let ctx = egui::Context::default();

docs/target-app-parity.md

@@ -1274,7 +1274,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   Delete, Cut, Group, Ungroup, Make and Release Compound Path, Make Clipping Mask, Send to Current Layer, Duplicate, Copy in the Move, Rotate, Scale, Reflect and Shear dialogs, Transform Each with Copy and each move of an Alt-drag take time in proportion to the size of the document (#1429).
   In the web build the status bar's render and UI times are measured with the browser's `performance.now()`, and the Layers panel stops rendering row thumbnails in a frame once they have taken 8 ms.
   On the desktop, the Layers panel renders a row's thumbnail on its thumbnail thread when the row's paths, and the paths of its opacity masks that are not disabled, hold more than 2,000 anchors in all. It already did this for the first thumbnail of a row of more than 200 objects. Until its first thumbnail is ready, the row shows boxes at the bounds of up to 41 of its paths, each in its path's solid fill or stroke color or in gray, when its own paths, without its opacity masks, hold at most 2,000 anchors in all, and an empty white box otherwise. With one path of 200,000 anchors, the first frame after the panel opens takes 2.4 ms instead of 393 ms (release build, median of three runs).
-  The same 2,000-anchor limit counts each glyph of type as 4 anchors, each point of a gradient mesh as 1 and every 100 pixels of an image as 1, and a symbol instance or a placed document counts the objects and anchors of the art it shows. A placed document whose file has not been read yet, or a symbol nested more than 8 deep (one that holds an instance of itself, for example), gets its first thumbnail on the thumbnail thread. The first frame after the panel opens takes 2.5 ms instead of 23.4 ms with 5,000 glyphs of point type, 2.1 ms instead of 346 ms with one instance of a symbol of a 200,000-anchor path, 1.7 ms instead of 11.7 ms with a 100 × 100 mesh, and 2.3 ms instead of 273 ms with a placed document of a 200,000-anchor path (release build, median of three runs).
+  The same 2,000-anchor limit counts each glyph of type as 4 anchors, each point of a gradient mesh as 1 and every 100 pixels of an image as 1, and a symbol instance or a placed document counts the objects and anchors of the art it shows. A placed document whose file has not been read yet, or a symbol nested more than 8 deep (one that holds an instance of itself, for example), gets its first thumbnail on the thumbnail thread, and so does a row whose art takes more than 20,000 steps to count (each object, subpath, text run and symbol looked at is one step). The first frame after the panel opens takes 2.4 ms instead of 19.8 ms with 5,000 glyphs of point type, 1.8 ms instead of 343 ms with one instance of a symbol of a 200,000-anchor path, 2.0 ms instead of 11.0 ms with a 100 × 100 mesh, and 2.1 ms instead of 274 ms with a placed document of a 200,000-anchor path (release build, median of three runs).
   In the web build, while a pan, zoom or rotation changes the view of a document whose last canvas render took more than 8 ms, the canvas draws the last frame moved with the view, and it renders the art again once the view has been still for 0.2 s.
   An Alt-drag with the Selection, Direct Selection, Group Selection, Rotate, Scale, Reflect or Shear tool copies the selection on its first move, and the moves after it transform those copies; the previews of the Move, Rotate, Scale, Reflect and Shear dialogs with Copy and Preview on copy the selection once in the same way (#1429).
   An Alt-drag of objects in perspective with the Perspective Selection tool, and an Alt-drag of a plane widget with that tool or the Perspective Grid tool, copy the objects on the first move, and the moves after it move those copies (#1429).