From ebc171be57b2aceba34f590960739891d8416fad Mon Sep 17 00:00:00 2001
From: "Dylan M. Taylor" <dylan@dylanmtaylor.com>
Date: Tue, 3 Nov 2026 11:26:59 +0000
Subject: [PATCH] engine: extend an ink stroke in place and cap a gesture's
 samples

---
 crates/engine/src/tests.rs         | 16 ++++++++++++++++
 crates/engine/src/tools.rs         |  1 +
 crates/engine/src/tools/drag.rs    | 24 ++++++++++++++++++------
 crates/engine/src/tools/pointer.rs |  4 +++-
 4 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 68de13418..b7d029f30 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -5023,6 +5023,22 @@ fn pen_pressure_is_kept_and_a_mouse_draws_at_full_width() {
     assert_eq!(got, vec![vec![0.2, 1.0, 0.4], vec![1.0, 1.0]]);
 }
 
+#[test]
+fn a_long_ink_stroke_is_extended_in_place_and_capped() {
+    let mut s = session();
+    s.set_tool(tools::ToolKind::Ink { mode: "pen".into(), color: deckcraft_model::Rgba::rgb(20, 20, 20), width: 4.0 });
+    let ev = |kind, x: f64| tools::PointerEvent { kind, x, y: 80.0, mods: Mods::default(), tol: 3.0, pressure: Some(0.5) };
+    s.pointer(ev(PointerKind::Down, 0.0)).unwrap();
+    let n = tools::MAX_GESTURE_POINTS + 5_000;
+    for i in 1..n {
+        s.pointer(ev(PointerKind::Drag, (i % 500) as f64)).unwrap();
+    }
+    assert_eq!(s.tool.ink_preview.len(), tools::MAX_GESTURE_POINTS);
+    s.pointer(ev(PointerKind::Up, 0.0)).unwrap();
+    let kept: usize = ink_pressures(&s).iter().map(Vec::len).sum();
+    assert!((2..=tools::MAX_GESTURE_POINTS).contains(&kept), "{kept}");
+}
+
 fn ink_pressures(s: &Session) -> Vec<Vec<f32>> {
     let st = s.doc().unwrap();
     let Some(sh) = st.shapes().iter().rev().find(|sh| matches!(sh.kind, deckcraft_model::ShapeKind::Ink { .. })) else {
diff --git a/crates/engine/src/tools.rs b/crates/engine/src/tools.rs
index 82f53dfc3..50a0fbb02 100644
--- a/crates/engine/src/tools.rs
+++ b/crates/engine/src/tools.rs
@@ -17,6 +17,7 @@ mod geometry;
 mod hit;
 mod pointer;
 
+pub(crate) use drag::MAX_GESTURE_POINTS;
 pub use hit::{line_xfrm, resize_box, table_rows, text_pos};
 
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
diff --git a/crates/engine/src/tools/drag.rs b/crates/engine/src/tools/drag.rs
index dcee9cf7d..0c80df052 100644
--- a/crates/engine/src/tools/drag.rs
+++ b/crates/engine/src/tools/drag.rs
@@ -12,9 +12,24 @@ struct MoveDrag {
     enter_text: Option<(ShapeId, Point)>,
 }
 
+/// Samples one ink or freeform gesture keeps; later pointer moves extend it no further.
+pub(crate) const MAX_GESTURE_POINTS: usize = 20_000;
+
 impl Session {
     /// Update the active drag interaction using the pointer position and modifiers.
     pub(super) fn drag(&mut self, p: Point, mods: Mods, tol: f64) -> Result<Value> {
+        // Ink grows by one sample per move: extend it (and its preview) in place rather than
+        // copying the whole stroke twice per event.
+        if matches!(self.tool.drag, Some(Drag::Ink { .. })) {
+            let sample = self.ink_sample(p);
+            if let Some(Drag::Ink { points }) = self.tool.drag.as_mut()
+                && points.len() < MAX_GESTURE_POINTS
+            {
+                points.push(sample);
+                self.tool.ink_preview.push(sample);
+            }
+            return Ok(Value::Null);
+        }
         let Some(drag) = self.tool.drag.clone() else { return Ok(Value::Null) };
         match drag {
             Drag::Guide { index } => {
@@ -181,7 +196,7 @@ impl Session {
             }
             Drag::Freeform { mode, mut points, pressed } => {
                 // Dragging scribbles (except for curves, which only take clicked points).
-                if pressed && mode != "curve" && points.last().is_none_or(|l| (*l - p).hypot() >= 1.5) {
+                if pressed && mode != "curve" && points.len() < MAX_GESTURE_POINTS && points.last().is_none_or(|l| (*l - p).hypot() >= 1.5) {
                     points.push(p);
                 }
                 let mut v = points.clone();
@@ -191,11 +206,8 @@ impl Session {
                 self.tool.path_preview = v;
                 self.tool.drag = Some(Drag::Freeform { mode, points, pressed });
             }
-            Drag::Ink { mut points } => {
-                points.push(self.ink_sample(p));
-                self.tool.ink_preview = points.clone();
-                self.tool.drag = Some(Drag::Ink { points });
-            }
+            // Handled above.
+            Drag::Ink { .. } => {}
         }
         Ok(Value::Null)
     }
diff --git a/crates/engine/src/tools/pointer.rs b/crates/engine/src/tools/pointer.rs
index d20ae9315..8433a998e 100644
--- a/crates/engine/src/tools/pointer.rs
+++ b/crates/engine/src/tools/pointer.rs
@@ -228,7 +228,9 @@ impl Session {
                 self.tool.drag = Some(Drag::Freeform { mode, points, pressed: false });
                 return self.finish_freeform(true);
             }
-            points.push(p);
+            if points.len() < super::MAX_GESTURE_POINTS {
+                points.push(p);
+            }
             self.tool.path_preview = points.clone();
             self.tool.drag = Some(Drag::Freeform { mode, points, pressed: true });
             return Ok(Value::Null);