diff --git a/crates/oa-desktop/src/behavior/scenario/spacing/controller/tests.rs b/crates/oa-desktop/src/behavior/scenario/spacing/controller/tests.rs
index 87d5f13e..1344207d 100644
--- a/crates/oa-desktop/src/behavior/scenario/spacing/controller/tests.rs
+++ b/crates/oa-desktop/src/behavior/scenario/spacing/controller/tests.rs
@@ -328,3 +328,71 @@ fn zoomed_out_message_tolerance_matches_actual_canvas_hit() {
     state.begin_spacing(doc.project(), event).unwrap();
     assert_eq!(state.spacing_token(), Some(token));
 }
+
+#[test]
+fn fractional_screen_packet_preserves_subpixel_motion_in_preview_and_finish() {
+    let (doc, owner, diagram, _, message) = fixture();
+    let original = doc.snapshot();
+    assert!(doc.project().diagrams[&diagram].layout.is_none());
+    let mut state = State::default();
+    let document = Id::new_v4();
+    state.enter_representation(document, owner, Representation::Saved { diagram });
+    state.presentation.viewport = Some(Size::new(900., 600.));
+    state.presentation.fit = false;
+    state.presentation.zoom = Some(0.5);
+    // 406 + 2^-15 puts the press just below the 512 screen-coordinate boundary.
+    state.presentation.pan = Vector::new(0., f32::from_bits(0x43cb_0001));
+    let camera = state.camera_boundary();
+    let context = state.context(document, owner, doc.revision(), 4).unwrap();
+    let baseline = resolve(doc.project(), context.representation, message).unwrap();
+    let line = baseline.scene.lines.iter().find(|line| line.id == message).unwrap();
+    assert_eq!(line.points[0].y, 148.);
+    let begin = packet(doc.project(), &state, context, 9, Target::Message(message));
+    let Event::Begin { press_screen, canvas_bounds, scale, pan, .. } = begin else {
+        unreachable!()
+    };
+    assert_eq!(canvas_bounds.y, 23.);
+    assert_eq!(scale, 0.5);
+    assert_eq!(pan.y.to_bits(), 0x43cb_0001);
+    assert_eq!(press_screen.y.to_bits(), 0x43fb_8001); // 503 + 2^-15.
+    let end = press_screen + Vector::new(0., 10.);
+    assert_eq!(end.y, 513.);
+    assert!(canvas_bounds.contains(press_screen) && canvas_bounds.contains(end));
+    // The submitted f32 displacement is 10 - 2^-15, not exactly 10.
+    assert_eq!((end.y - press_screen.y).to_bits(), 0x411f_ffe0);
+    assert_eq!(((end.y - press_screen.y) / scale).to_bits(), 0x419f_ffe0);
+    // Independent oracle: 148 + (10 - 2^-15) / .5 = 168 - 2^-14.
+    let expected_y = f32::from_bits(0x4327_fffc);
+    let mut expected = original.decoded.project.clone();
+    expected.diagrams.get_mut(&diagram).unwrap().layout = Some(
+        oa_core::DiagramLayout::Scenario(oa_core::ScenarioLayout {
+            lanes: vec![],
+            messages: vec![oa_core::ScenarioMessagePosition {
+                address: oa_core::OccurrenceEndpoint {
+                    references: vec![],
+                    occurrence: message,
+                },
+                send_y: expected_y,
+            }],
+        }),
+    );
+    state.begin_spacing(doc.project(), begin).unwrap();
+    state.move_spacing(end);
+    let preview = state.spacing.as_ref().unwrap().preview.as_ref().unwrap();
+    assert_eq!(preview.project, expected);
+    let line = preview.geometry.scene.lines.iter().find(|line| line.id == message).unwrap();
+    assert_eq!(line.points[0].y.to_bits(), expected_y.to_bits());
+    let Finish::Commit { command, selected, scale, pan } = state.finish_spacing(end).unwrap() else {
+        panic!("fractional motion must remain a commit")
+    };
+    assert_eq!(selected, message);
+    assert_eq!(scale, 0.5);
+    assert_eq!(pan.y.to_bits(), 0x43cb_0001);
+    let committed = oa_core::execute_batch(doc.project(), &[command]).unwrap();
+    assert_eq!(committed, expected);
+    assert!(state.spacing_token().is_none());
+    assert_eq!(doc.project(), &original.decoded.project);
+    assert_eq!(doc.compiled().bundle, original.bundle);
+    assert!(!doc.dirty() && !doc.can_undo() && !doc.can_redo());
+    assert!(camera == state.camera_boundary());
+}
diff --git a/crates/oa-desktop/src/behavior/scenario/workspace/spacing_tests.rs b/crates/oa-desktop/src/behavior/scenario/workspace/spacing_tests.rs
index 3f50d198..5f3bd640 100644
--- a/crates/oa-desktop/src/behavior/scenario/workspace/spacing_tests.rs
+++ b/crates/oa-desktop/src/behavior/scenario/workspace/spacing_tests.rs
@@ -156,8 +156,26 @@ async fn saved_message_same_batch_pointer_events_commit_once_and_one_undo_redo_r
     let target = id(&app, "updated_report");
     let before = app.document.snapshot();
     let revision = app.document.revision();
-    let start = screen(&mut mounted, &app, Point::new(400., 252.));
+    let projected = screen(&mut mounted, &app, Point::new(400., 252.));
+    // This fixture asks for exactly +20 world units. A fractional start near a
+    // screen-coordinate exponent boundary need not retain an exact +10 delta.
+    let start = Point::new(projected.x.round(), projected.y.round());
+    let mut geometry = resolved(&app);
+    let camera = app.behavior.scenario.presentation.camera(geometry.bounds);
+    assert_eq!(camera.scale, 0.5);
+    let bounds = mounted.geometry(&app).bounds("scenario-viewport");
+    let world = Point::new(
+        (start.x - bounds.x - camera.pan.x) / camera.scale,
+        (start.y - bounds.y - camera.pan.y) / camera.scale,
+    );
+    geometry.scene.scale = camera.scale;
+    geometry.scene.camera_pan = camera.pan;
+    let hit = geometry.scene.hit_item(world).unwrap();
+    assert_eq!(hit.id, target);
+    assert_eq!(hit.role, crate::behavior::SceneHitRole::MessageBody);
     let end = start + Vector::new(0., 10.); // .5 display scale: +20 authored y.
+    assert_eq!(end.y - start.y, 10.);
+    assert!(bounds.contains(start) && bounds.contains(end));
     let messages = mounted
         .batch(
             &mut app,