fix: load designs off-thread and cache recent thumbnails as PNGs

crates/engine/src/lib.rs

@@ -439,6 +439,42 @@ impl Session {
         }
     }
 
+    /// Refresh UI caches when adopting a session or switching tabs, without discarding its
+    /// already prepared world-space geometry. This does not change the document.
+    pub fn invalidate_view(&mut self, previous_revision: u64) {
+        let old = self.revision;
+        self.revision = old.max(previous_revision) + 1;
+        if let Ok(mut cache) = self.world_cache.lock()
+            && let Some((revision, _)) = cache.as_mut()
+            && *revision == old
+        {
+            *revision = self.revision;
+        }
+    }
+
+    /// Commit commands evaluated on a scratch copy without losing this session's file,
+    /// saved state, view state, checkpoints or earlier undo steps.
+    pub fn accept_computed(&mut self, mut computed: Session) {
+        let previous_revision = self.revision;
+        self.doc = computed.doc;
+        self.model = computed.model;
+        self.world_cache = computed.world_cache;
+        self.undo.append(&mut computed.undo);
+        if self.undo.len() > MAX_UNDO {
+            self.undo.drain(..self.undo.len() - MAX_UNDO);
+        }
+        self.redo = computed.redo;
+        self.active_sketch = computed.active_sketch;
+        self.selection = computed.selection;
+        self.revision = computed.revision;
+        self.invalidate_view(previous_revision);
+        self.active_component = computed.active_component;
+        self.pending_moves = computed.pending_moves;
+        for message in computed.log {
+            self.echo(message);
+        }
+    }
+
     /// Live preview: run `commands` (id, parameters) on a scratch copy and return the model
     /// before and after. Nothing in this session changes (document, undo history, selection).
     /// A `timeline.redefine` is shown even while the timeline is rolled back to the feature

crates/engine/src/tests.rs

@@ -2032,3 +2032,40 @@ fn drawing_on_existing_points_connects_to_them() {
     let si = run(&mut s, "sketch.inspect", json!({}));
     assert_ne!(si["status"], "conflict", "{si}");
 }
+
+#[test]
+fn computed_import_preserves_file_and_earlier_undo_history() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    s.path = Some("existing.solvecraft".into());
+    s.mark_saved();
+    let saved = s.doc.clone();
+    let mut computed = s.scratch();
+    run(&mut computed, "solid.box", json!({"length": 5, "width": 5, "height": 5, "corner": [30, 0, 0]}));
+    s.accept_computed(computed);
+    assert_eq!(s.path.as_deref(), Some("existing.solvecraft"));
+    assert!(s.is_dirty());
+    assert_eq!(s.model.state().bodies.len(), 2);
+    run(&mut s, "edit.undo", json!({}));
+    assert!(std::sync::Arc::ptr_eq(&saved, &s.doc));
+    assert!(!s.is_dirty());
+    run(&mut s, "edit.undo", json!({}));
+    assert!(s.model.state().bodies.is_empty());
+}
+
+#[test]
+fn adopting_a_computed_component_keeps_its_warm_world_geometry() {
+    let mut s = Session::default();
+    run(&mut s, "component.create", json!({"name": "Part"}));
+    run(&mut s, "solid.box", json!({"length":10,"width":10,"height":10}));
+    let mut computed = s.scratch();
+    run(&mut computed, "solid.box", json!({"length":5,"width":5,"height":5,"corner":[30,0,0]}));
+    let prepared = computed.world_state();
+    for b in &prepared.bodies {
+        let _ = b.mesh();
+    }
+    s.accept_computed(computed);
+    assert!(std::sync::Arc::ptr_eq(&prepared, &s.world_state()));
+    s.invalidate_view(100);
+    assert!(std::sync::Arc::ptr_eq(&prepared, &s.world_state()));
+}