From ba7e3439f4f49e4c9a9467d87af9332ecf46aad5 Mon Sep 17 00:00:00 2001
From: Ryan Di <ryan.weihao.di@gmail.com>
Date: Fri, 28 Feb 2025 15:41:35 +1100
Subject: [PATCH] add a tiny threshold for checks

---
 packages/excalidraw/lasso/worker.ts |  3 ++-
 packages/math/segment.ts            | 10 +++++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/packages/excalidraw/lasso/worker.ts b/packages/excalidraw/lasso/worker.ts
index aa1379c90..74290a9a9 100644
--- a/packages/excalidraw/lasso/worker.ts
+++ b/packages/excalidraw/lasso/worker.ts
@@ -166,7 +166,8 @@ const intersectionTest = (
   return lassoSegments.some((lassoSegment) =>
     elementSegments.some(
       (elementSegment) =>
-        lineSegmentIntersectionPoints(lassoSegment, elementSegment) !== null,
+        // introduce a bit of tolerance to account for roughness and simplification of paths
+        lineSegmentIntersectionPoints(lassoSegment, elementSegment, 1) !== null,
     ),
   );
 };
diff --git a/packages/math/segment.ts b/packages/math/segment.ts
index 60943aacb..608342471 100644
--- a/packages/math/segment.ts
+++ b/packages/math/segment.ts
@@ -159,13 +159,17 @@ export const distanceToLineSegment = <Point extends LocalPoint | GlobalPoint>(
  */
 export function lineSegmentIntersectionPoints<
   Point extends GlobalPoint | LocalPoint,
->(l: LineSegment<Point>, s: LineSegment<Point>): Point | null {
+>(
+  l: LineSegment<Point>,
+  s: LineSegment<Point>,
+  threshold?: number,
+): Point | null {
   const candidate = linesIntersectAt(line(l[0], l[1]), line(s[0], s[1]));
 
   if (
     !candidate ||
-    !pointOnLineSegment(candidate, s) ||
-    !pointOnLineSegment(candidate, l)
+    !pointOnLineSegment(candidate, s, threshold) ||
+    !pointOnLineSegment(candidate, l, threshold)
   ) {
     return null;
   }