From a712786ecf5f39d89496ee529cbfda65dde37359 Mon Sep 17 00:00:00 2001
From: atagen <boss@atagen.co>
Date: Thu, 21 May 2026 19:48:13 +1000
Subject: [PATCH] Choose swap lanes by motion direction

---
 docs/window-animations-plan.md |  3 ++
 src/animation/multiphase.rs    | 69 ++++++++++++++++++++++++++++++++--
 2 files changed, 69 insertions(+), 3 deletions(-)

diff --git a/docs/window-animations-plan.md b/docs/window-animations-plan.md
index a659d76a..3a79fafd 100644
--- a/docs/window-animations-plan.md
+++ b/docs/window-animations-plan.md
@@ -217,6 +217,9 @@ Preferred approach:
 Current pure planner status:
 
 - Two-window same-axis swaps use shrink lanes, move, then grow.
+- Swap lane choice follows motion direction, not node identity: right/down
+  moving windows take the first lane, and left/up moving windows take the second
+  lane.
 - Stack extraction/return patterns are covered in both horizontal and vertical
   orientations: peer/container space scales first, the extracted child moves
   only after space exists, and orthogonal growth happens in the final phase.
diff --git a/src/animation/multiphase.rs b/src/animation/multiphase.rs
index 37b31619..3a259359 100644
--- a/src/animation/multiphase.rs
+++ b/src/animation/multiphase.rs
@@ -173,7 +173,12 @@ fn plan_axis_crossing_lanes(
     }
 
     let mut windows = request.windows.clone();
-    windows.sort_by_key(|window| window.node_id.0);
+    windows.sort_by_key(|window| lane_index_for_direction(*window, axis));
+    if windows.windows(2).any(|pair| {
+        lane_index_for_direction(pair[0], axis) == lane_index_for_direction(pair[1], axis)
+    }) {
+        return None;
+    }
     let mut phase1 = vec![];
     let mut phase2 = vec![];
     let mut phase3 = vec![];
@@ -205,6 +210,15 @@ fn plan_axis_crossing_lanes(
     )
 }
 
+fn lane_index_for_direction(window: MultiphaseWindow, axis: PhaseAxis) -> Option<usize> {
+    let delta = main_start(window.to, axis) - main_start(window.from, axis);
+    match delta.cmp(&0) {
+        std::cmp::Ordering::Greater => Some(0),
+        std::cmp::Ordering::Less => Some(1),
+        std::cmp::Ordering::Equal => None,
+    }
+}
+
 fn plan_space_then_orthogonal_growth(
     request: &MultiphaseRequest,
     axis: PhaseAxis,
@@ -627,6 +641,15 @@ mod tests {
         plan.phases.iter().map(|phase| phase.action).collect()
     }
 
+    fn step_to(plan: &MultiphasePlan, phase: usize, node_id: NodeId) -> Rect {
+        plan.phases[phase]
+            .steps
+            .iter()
+            .find(|step| step.node_id == node_id)
+            .unwrap()
+            .to
+    }
+
     #[test]
     fn horizontal_swap_shrinks_moves_then_grows_without_overlap() {
         let req = request(vec![
@@ -679,8 +702,48 @@ mod tests {
             },
         ]);
         let plan = plan_no_overlap(&req).unwrap();
-        assert_eq!(plan.phases[0].steps[0].to, rect(100, 0, 200, 50));
-        assert_eq!(plan.phases[0].steps[1].to, rect(0, 50, 100, 100));
+        assert_eq!(step_to(&plan, 0, id(2)), rect(0, 0, 100, 50));
+        assert_eq!(step_to(&plan, 0, id(1)), rect(100, 50, 200, 100));
+        assert!(validate_plan_continuous(&req, &plan));
+    }
+
+    #[test]
+    fn horizontal_swap_lanes_follow_motion_direction_not_node_id() {
+        let req = request(vec![
+            MultiphaseWindow {
+                node_id: id(1),
+                from: rect(100, 0, 200, 100),
+                to: rect(0, 0, 100, 100),
+            },
+            MultiphaseWindow {
+                node_id: id(2),
+                from: rect(0, 0, 100, 100),
+                to: rect(100, 0, 200, 100),
+            },
+        ]);
+        let plan = plan_no_overlap(&req).unwrap();
+        assert_eq!(step_to(&plan, 0, id(2)), rect(0, 0, 100, 50));
+        assert_eq!(step_to(&plan, 0, id(1)), rect(100, 50, 200, 100));
+        assert!(validate_plan_continuous(&req, &plan));
+    }
+
+    #[test]
+    fn vertical_swap_lanes_follow_motion_direction_not_node_id() {
+        let req = request(vec![
+            MultiphaseWindow {
+                node_id: id(1),
+                from: rect(0, 100, 100, 200),
+                to: rect(0, 0, 100, 100),
+            },
+            MultiphaseWindow {
+                node_id: id(2),
+                from: rect(0, 0, 100, 100),
+                to: rect(0, 100, 100, 200),
+            },
+        ]);
+        let plan = plan_no_overlap(&req).unwrap();
+        assert_eq!(step_to(&plan, 0, id(2)), rect(0, 0, 50, 100));
+        assert_eq!(step_to(&plan, 0, id(1)), rect(50, 100, 100, 200));
         assert!(validate_plan_continuous(&req, &plan));
     }