Fix crash with zero length arrow (#2173)

This PR fixes a bug with zero length arrows.

Before:

![2023-11-07 at 15 17 47 - Harlequin
Mouse](https://github.com/tldraw/tldraw/assets/15892272/4c0b9609-f4f0-49cc-8575-ffd58973e81f)

After:

![2023-11-07 at 15 30 07 - Scarlet
Dinosaur](https://github.com/tldraw/tldraw/assets/15892272/af3188f1-c996-46e1-a2ff-48de31e0fc05)

Fixes #2166

### Change Type

- [x] `patch` — Bug fix

[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version

### Test Plan

1. Draw a rectangle.
2. Clone it.
3. Connect their centers with an arrow.
4. Hold control/command.
5. Drag one rectangle onto the other so they're in the exact same place.
6. Erase both rectangles (but NOT the invisible arrow).
7. Select all.
8. Hover the minimap.
9. Repeat but curve the arrow slightly before moving the rectangles
together.

- [ ] Unit Tests
- [ ] End to end tests

### Release Notes

- Fix a hyper niche arrow crash with zero length arrows.

packages/editor/src/lib/editor/shapes/shared/arrow/curved-arrow.ts

@@ -49,6 +49,25 @@ export function getCurvedArrowInfo(
 	const b = terminalsInArrowSpace.end.clone()
 	const c = middle.clone()
 
+	if (Vec2d.Equals(a, b)) {
+		return {
+			isStraight: true,
+			start: {
+				handle: a,
+				point: a,
+				arrowhead: shape.props.arrowheadStart,
+			},
+			end: {
+				handle: b,
+				point: b,
+				arrowhead: shape.props.arrowheadEnd,
+			},
+			middle: c,
+			isValid: false,
+			length: 0,
+		}
+	}
+
 	const isClockwise = shape.props.bend < 0
 	const distFn = isClockwise ? clockwiseAngleDist : counterClockwiseAngleDist
 

packages/editor/src/lib/editor/shapes/shared/arrow/straight-arrow.ts

@@ -25,6 +25,26 @@ export function getStraightArrowInfo(editor: Editor, shape: TLArrowShape): TLArr
 	const a = terminalsInArrowSpace.start.clone()
 	const b = terminalsInArrowSpace.end.clone()
 	const c = Vec2d.Med(a, b)
+
+	if (Vec2d.Equals(a, b)) {
+		return {
+			isStraight: true,
+			start: {
+				handle: a,
+				point: a,
+				arrowhead: shape.props.arrowheadStart,
+			},
+			end: {
+				handle: b,
+				point: b,
+				arrowhead: shape.props.arrowheadEnd,
+			},
+			middle: c,
+			isValid: false,
+			length: 0,
+		}
+	}
+
 	const uAB = Vec2d.Sub(b, a).uni()
 
 	// Update the arrowhead points using intersections with the bound shapes, if any.

packages/tldraw/api/api.json

@@ -14583,7 +14583,7 @@
               "text": "export interface TLUiContextMenuProps "
             }
           ],
-          "fileUrlPath": "packages/tldraw/src/lib/ui/components/ContextMenu.tsx",
+          "fileUrlPath": "packages/tldraw/.tsbuild-api/lib/ui/components/ContextMenu.d.ts",
           "releaseTag": "Public",
           "name": "TLUiContextMenuProps",
           "preserveMemberOrder": false,
@@ -14606,6 +14606,7 @@
                   "text": ";"
                 }
               ],
+              "fileUrlPath": "packages/tldraw/src/lib/ui/components/ContextMenu.tsx",
               "isReadonly": false,
               "isOptional": false,
               "releaseTag": "Public",