Fix live hinge animation and Drive Joint slider previews (#835)

Joint previews keep showing finished poses while the slider moves, and the latest value is still computed.

crates/ui-egui/src/dialogs_assembly_tests.rs

@@ -264,6 +264,69 @@ fn drive_value_takes_the_keyboard() {
     assert!(app.dialog.is_none());
 }
 
+/// A real slider drag updates the requested pose while the pointer is still down.
+/// Previewing and cancelling must leave the saved joint and occurrence positions alone.
+#[test]
+fn drive_slider_previews_intermediate_positions_before_release() {
+    let mut h = crate::scenario::Harness::new();
+    h.app = two_boxes();
+    h.app.viewport.no_pixels = true;
+    let (a, b) = (occ(&h.app, "A"), occ(&h.app, "B"));
+    h.app
+        .run(
+            "joint.create",
+            json!({"type": "revolute", "a": {"occurrence": a, "face": [10, 10, 10]},
+        "b": {"occurrence": b, "face": [40, 10, 0]}, "limits": [[-90, 90]]}),
+        )
+        .unwrap();
+    let before = h.app.session.doc.clone();
+    h.app.start("joint.drive");
+    h.frames(4);
+    let bounds = h
+        .output
+        .accesskit_update
+        .as_ref()
+        .unwrap()
+        .nodes
+        .iter()
+        .find_map(|(_, n)| (n.role() == egui::accesskit::Role::Slider).then(|| n.bounds()).flatten())
+        .expect("drive slider");
+    let at = |fraction: f64| egui::pos2((bounds.x0 + fraction * (bounds.x1 - bounds.x0)) as f32, ((bounds.y0 + bounds.y1) / 2.0) as f32);
+    let start = at(0.25);
+    h.app.synthetic.extend([
+        egui::Event::PointerMoved(start),
+        egui::Event::PointerButton { pos: start, button: egui::PointerButton::Primary, pressed: true, modifiers: Default::default() },
+    ]);
+    h.frames(3);
+    let first = preview_commands(&h.app, h.app.dialog.as_ref().unwrap()).unwrap();
+    h.app.synthetic.push(egui::Event::PointerMoved(at(0.75)));
+    h.frames(3);
+    let second = preview_commands(&h.app, h.app.dialog.as_ref().unwrap()).unwrap();
+    assert_ne!(first, second, "the held slider requests a new pose before release");
+    assert!(h.ctx.input(|i| i.pointer.primary_down()));
+    let deadline = std::time::Instant::now() + std::time::Duration::from_secs(10);
+    while h.app.preview.busy {
+        assert!(std::time::Instant::now() < deadline, "held-slider preview did not finish");
+        std::thread::sleep(std::time::Duration::from_millis(1));
+        h.frame();
+    }
+    assert!(h.app.preview.active, "the viewport shows the pose before release");
+    assert!(h.app.preview.error.is_none());
+    assert_eq!(h.app.preview.replaced, ["Body2"]);
+    assert_eq!(h.app.session.doc, before);
+    h.app.synthetic.push(egui::Event::PointerButton {
+        pos: at(0.75),
+        button: egui::PointerButton::Primary,
+        pressed: false,
+        modifiers: Default::default(),
+    });
+    h.frames(3);
+    h.step(&json!({"key": "Escape"})).unwrap();
+    assert!(h.app.dialog.is_none());
+    assert!(!h.app.preview.active);
+    assert_eq!(h.app.session.doc, before, "cancel leaves the joint at its original angle");
+}
+
 /// Named joint origins are drawn where their component is: a drive of the occurrence, or a free
 /// move not captured yet, carries them along.
 #[test]