Make the Pen tool extend an endpoint by starting with a colinear, equidistant handle (#2295)

* Fixed Pen extension having collienar handles

* Reformat a little

* handles being colinear for GRS

---------

Co-authored-by: Keavon Chambers <keavon@keavon.com>

editor/src/messages/tool/tool_messages/pen_tool.rs

@@ -690,25 +690,9 @@ impl PenToolData {
 		self.handle_end = None;
 
 		let tolerance = crate::consts::SNAP_POINT_TOLERANCE;
-		if let Some((layer, point, position)) = should_extend(document, viewport, tolerance, selected_nodes.selected_layers(document.metadata()), preferences) {
-			// Perform extension of an existing path
-			let in_segment = if self.modifiers.lock_angle { self.end_point_segment } else { None };
-			self.add_point(LastPoint {
-				id: point,
-				pos: position,
-				in_segment,
-				handle_start: position,
-			});
-			responses.add(NodeGraphMessage::SelectedNodesSet { nodes: vec![layer.to_node()] });
-			self.next_point = position;
-			self.next_handle_start = position;
-			let vector_data = document.network_interface.compute_modified_vector(layer).unwrap();
-			let segment = vector_data.all_connected(point).collect::<Vec<_>>().first().map(|s| s.segment);
-
-			if self.modifiers.lock_angle {
-				self.set_lock_angle(&vector_data, point, segment);
-			}
-
+		let extension_choice = should_extend(document, viewport, tolerance, selected_nodes.selected_layers(document.metadata()), preferences);
+		if let Some((layer, point, position)) = extension_choice {
+			self.extend_existing_path(document, layer, point, position, responses);
 			return;
 		}
 
@@ -759,6 +743,64 @@ impl PenToolData {
 		responses.add(PenToolMessage::AddPointLayerPosition { layer, viewport });
 	}
 
+	/// Perform extension of an existing path
+	fn extend_existing_path(&mut self, document: &DocumentMessageHandler, layer: LayerNodeIdentifier, point: PointId, position: DVec2, responses: &mut VecDeque<Message>) {
+		let vector_data = document.network_interface.compute_modified_vector(layer);
+		let (handle_start, in_segment) = if let Some(vector_data) = &vector_data {
+			vector_data
+				.segment_bezier_iter()
+				.find_map(|(segment_id, bezier, start, end)| {
+					let is_end = point == end;
+					let is_start = point == start;
+					if !is_end && !is_start {
+						return None;
+					}
+
+					let handle = match bezier.handles {
+						BezierHandles::Cubic { handle_start, handle_end, .. } => {
+							if is_start {
+								handle_start
+							} else {
+								handle_end
+							}
+						}
+						BezierHandles::Quadratic { handle } => handle,
+						_ => return None,
+					};
+					Some((segment_id, is_end, handle))
+				})
+				.map(|(segment_id, is_end, handle)| {
+					let mirrored_handle = position * 2. - handle;
+					let in_segment = if is_end { Some(segment_id) } else { None };
+					(mirrored_handle, in_segment)
+				})
+				.unwrap_or_else(|| (position, None))
+		} else {
+			(position, None)
+		};
+
+		let in_segment = if self.modifiers.lock_angle { self.end_point_segment } else { in_segment };
+
+		self.add_point(LastPoint {
+			id: point,
+			pos: position,
+			in_segment,
+			handle_start,
+		});
+
+		responses.add(NodeGraphMessage::SelectedNodesSet { nodes: vec![layer.to_node()] });
+
+		self.next_point = position;
+		self.next_handle_start = handle_start;
+		let vector_data = document.network_interface.compute_modified_vector(layer).unwrap();
+		let segment = vector_data.all_connected(point).collect::<Vec<_>>().first().map(|s| s.segment);
+
+		if self.modifiers.lock_angle {
+			self.set_lock_angle(&vector_data, point, segment);
+		}
+		self.handle_mode = HandleMode::ColinearEquidistant;
+	}
+
 	// Stores the segment and point ID of the clicked endpoint
 	fn store_clicked_endpoint(&mut self, document: &DocumentMessageHandler, input: &InputPreprocessorMessageHandler, preferences: &PreferencesMessageHandler) {
 		let point = SnapCandidatePoint::handle(document.metadata().document_to_viewport.inverse().transform_point2(input.mouse.position));