Blend: Escape drops the Blend tool's first pick

After a click picked the first object with the Blend tool, Escape did
nothing: the pick stayed, and the next click on another object blended
the two. After a blend was made, Escape did not stop the next click
adding to it either.

Escape now drops the object picked first and stops the next click
adding to the blend just made, as a click on empty canvas does, so the
next click picks a new first object. The document and the selection
stay as they are, and no undo step is added.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/engine/src/tests/blendopts.rs

@@ -1,10 +1,11 @@
 //! Blend Options (defaults with nothing selected, `object.blend.info`) and the Blend tool:
-//! start points from clicked anchors, adding keys to a blend instead of nesting it (M8.18).
+//! start points from clicked anchors, adding keys to a blend instead of nesting it (M8.18), and
+//! Escape dropping the first pick.
 
 use serde_json::{Value, json};
 use vectorcraft_doc::live::{BlendOrientation, BlendSpacing, BlendSpec};
 use vectorcraft_doc::{Node, NodeKind};
-use vectorcraft_tools::{Mods, PointerEvent, PointerKind};
+use vectorcraft_tools::{Mods, PointerEvent, PointerKind, ToolKey};
 
 use crate::*;
 
@@ -140,6 +141,32 @@ fn the_blend_tool_blends_from_clicked_anchors_and_keeps_adding() {
     assert!(matches!(r.unwrap().as_slice(), [UiRequest::Dialog(k, _)] if k == "blendOptions"));
 }
 
+#[test]
+fn escape_drops_the_blend_tools_first_pick() {
+    let mut s = session();
+    let (a, b, c) = (rect(&mut s, 0.0, 0.0, 40.0), rect(&mut s, 200.0, 0.0, 40.0), rect(&mut s, 400.0, 0.0, 40.0));
+    s.select_tool("blend", ViewInfo::default()).unwrap();
+    let click = |s: &mut Session, x: f64, y: f64| s.pointer(&PointerEvent::new(PointerKind::Down, x, y), ViewInfo::default()).unwrap();
+    let esc = |s: &mut Session| s.tool_key(ToolKey::Escape, Mods::default(), ViewInfo::default()).unwrap();
+    click(&mut s, 20.0, 20.0);
+    assert!(s.tool_busy());
+    let (layers, depth) = (s.doc().unwrap().doc.layers.clone(), s.doc().unwrap().history.undo.len());
+    assert!(esc(&mut s).is_empty());
+    assert!(!s.tool_busy(), "Escape drops the pick");
+    assert_eq!((&s.doc().unwrap().doc.layers, s.doc().unwrap().history.undo.len()), (&layers, depth), "and changes nothing");
+    // The next two clicks blend b and c; a is not part of it.
+    click(&mut s, 220.0, 20.0);
+    assert_eq!(s.doc().unwrap().doc.layers, layers, "b is picked first");
+    click(&mut s, 420.0, 20.0);
+    let g = s.doc().unwrap().selection.objects[0];
+    assert_eq!(spec(&s, g).0, vec![b, c]);
+    // Escape after the blend is made: the next click starts a new blend instead of adding to it.
+    esc(&mut s);
+    click(&mut s, 20.0, 20.0);
+    assert_eq!(s.doc().unwrap().selection.objects, vec![a]);
+    assert_eq!(spec(&s, g).0, vec![b, c], "a was not added");
+}
+
 #[test]
 fn start_points_round_trip_and_old_blends_load() {
     let mut s = session();