[fix] Hit testing against zero width / height lines (#2060)

This PR fixes a bug where certain hit tests would always miss straight
lines.

### Change Type

- [x] `patch` — Bug fix

### Test Plan

1. Create a straight line.
2. Try to bind an arrow to it.

- [x] Unit Tests

### Release Notes

- [fix] Bug where arrows would not bind to straight lines

packages/editor/src/lib/editor/Editor.ts

@@ -4319,9 +4319,21 @@ export class Editor extends EventEmitter<TLEventMap> {
 
 				distance = minDistance
 			} else {
-				distance = geometry.bounds.containsPoint(pointInShapeSpace, margin)
-					? geometry.distanceToPoint(pointInShapeSpace, hitInside)
-					: Infinity
+				// If the margin is zero and the geometry has a very small width or height,
+				// then check the actual distance. This is to prevent a bug where straight
+				// lines would never pass the broad phase (point-in-bounds) check.
+				if (margin === 0 && (geometry.bounds.w < 1 || geometry.bounds.h < 1)) {
+					distance = geometry.distanceToPoint(pointInShapeSpace, hitInside)
+				} else {
+					// Broad phase
+					if (geometry.bounds.containsPoint(pointInShapeSpace, margin)) {
+						// Narrow phase (actual distance)
+						distance = geometry.distanceToPoint(pointInShapeSpace, hitInside)
+					} else {
+						// Failed the broad phase, geddafugaotta'ere!
+						distance = Infinity
+					}
+				}
 			}
 
 			if (geometry.isClosed) {

packages/tldraw/src/lib/shapes/arrow/ArrowShapeTool.test.ts

@@ -1,4 +1,4 @@
-import { Vec2d, createShapeId } from '@tldraw/editor'
+import { TLArrowShape, Vec2d, createShapeId } from '@tldraw/editor'
 import { TestEditor } from '../../../test/TestEditor'
 
 let editor: TestEditor
@@ -523,5 +523,29 @@ describe('line bug', () => {
 			.keyUp('Shift')
 
 		expect(editor.currentPageShapes.length).toBe(2)
+		const arrow = editor.currentPageShapes[1] as TLArrowShape
+		expect(arrow.props.end.type).toBe('binding')
+	})
+
+	it('works as expected when binding to a straight horizontal line', () => {
+		editor.selectAll().deleteShapes(editor.selectedShapeIds)
+
+		expect(editor.currentPageShapes.length).toBe(0)
+
+		editor
+			.setCurrentTool('line')
+			.pointerMove(0, 0)
+			.pointerDown()
+			.pointerMove(0, 100)
+			.pointerUp()
+			.setCurrentTool('arrow')
+			.pointerMove(50, 50)
+			.pointerDown()
+			.pointerMove(0, 50)
+			.pointerUp()
+
+		expect(editor.currentPageShapes.length).toBe(2)
+		const arrow = editor.currentPageShapes[1] as TLArrowShape
+		expect(arrow.props.end.type).toBe('binding')
 	})
 })