Do not journal a failed live preview at commit (#1720)

crates/engine/src/lib.rs

@@ -1573,13 +1573,17 @@ impl Session {
                 st.selection.clone_from(&it.selection);
             }
         }
+        // The previous preview's paint is undone with it.
+        self.pending_paint = None;
         let rw = cmd::distortcmds::perspective_rewrite(self, cmd, params);
         let (cmd, params) = rw.as_ref().map_or((cmd, params), |(c, p)| (c.as_str(), p));
         let r = self.execute(cmd, params);
         let params = self.noted(params);
         let st = self.doc_mut()?;
         if let Some(it) = &mut st.interaction {
-            it.preview = Some((cmd.to_string(), params));
+            // A failed preview leaves the document at the snapshot, so nothing is left to commit
+            // (not even an earlier successful preview, which the reset above already undid).
+            it.preview = r.is_ok().then(|| (cmd.to_string(), params));
         }
         r
     }

crates/engine/src/tests_journal.rs

@@ -371,3 +371,25 @@ fn a_move_drag_is_named_for_whether_it_copied() {
         assert_eq!(last(&s).as_deref(), Some(label), "begun as {begin}, copy: {copy}");
     }
 }
+
+#[test]
+fn a_failed_preview_is_not_journaled_at_commit() {
+    let mut s = Session::new();
+    s.execute("file.new", &json!({"width": 400, "height": 300})).unwrap();
+    s.execute("shape.rectangle", &json!({"x": 10, "y": 10, "width": 100, "height": 50})).unwrap();
+    let journal = s.journal.len();
+
+    // Only a failing preview: committing records nothing.
+    s.begin_interaction("Bad").unwrap();
+    assert!(s.preview("no.such.command", &json!({})).is_err());
+    s.commit_interaction().unwrap();
+    assert_eq!(s.journal.len(), journal);
+
+    // A good preview followed by a failing one: the document is back at the snapshot, so the
+    // earlier preview is not journaled either.
+    s.begin_interaction("Opacity").unwrap();
+    s.preview("transparency.set", &json!({"opacity": 50})).unwrap();
+    assert!(s.preview("no.such.command", &json!({})).is_err());
+    s.commit_interaction().unwrap();
+    assert_eq!(s.journal.len(), journal);
+}

crates/engine/src/tests_paintproxy.rs

@@ -210,6 +210,11 @@ fn live_previews_remember_only_the_committed_colour() {
     s.preview("paint.setFill", &json!({"color": "#400000"})).unwrap();
     s.cancel_interaction().unwrap();
     assert_eq!(hexes(&s), ["#300000"], "a cancelled preview is forgotten");
+    s.begin_interaction("Color").unwrap();
+    s.preview("paint.setFill", &json!({"color": "#500000"})).unwrap();
+    assert!(s.preview("no.such.command", &json!({})).is_err());
+    s.commit_interaction().unwrap();
+    assert_eq!(hexes(&s), ["#300000"], "a preview undone by a failed one is forgotten");
 }
 
 #[test]