From 733d0da49a17dedd418ceebee46dea90c94f3ca5 Mon Sep 17 00:00:00 2001
From: Nixon <43715558+nixonyh@users.noreply.github.com>
Date: Fri, 5 Jun 2026 00:43:41 +0800
Subject: [PATCH] Fix translation seed for non-root layout

`layout` seeded `propagate_translation` with the node's own world_translation, so laying out a non-root node re-added its own translation each pass, making it drift by its offset on every layout. This was masked for root layout, where both world and translation are zero.

Seed from the parent's world translation instead (zero when the node has no parent). Also prefix the `Id is invalid!` panics with the originating function name.
---
 crates/rectree/src/lib.rs | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/crates/rectree/src/lib.rs b/crates/rectree/src/lib.rs
index f12db47..1f4a6d4 100644
--- a/crates/rectree/src/lib.rs
+++ b/crates/rectree/src/lib.rs
@@ -194,7 +194,7 @@ pub fn layout<
     nodes: &mut N,
     id: &Id,
 ) {
-    let node = nodes.get_node(id).expect("Id is invalid!");
+    let node = nodes.get_node(id).expect("layout: Id is invalid!");
 
     let old_size = node.size;
     let parent = node.parent_id;
@@ -219,11 +219,15 @@ pub fn layout<
         bubbled_id = build_up(tree, nodes, parent_id);
     }
 
-    // 3. Propagate translation.
+    // 3. Propagate translation, seeding from `bubbled_id`'s parent
+    // world translation. Seeding from `bubbled_id`'s own translation
+    // would offset a non-root node by its own translation each pass;
+    // the root has no parent and seeds from zero.
     let parent_world = nodes
         .get_node(&bubbled_id)
-        .expect("Id is invalid!")
-        .world_translation;
+        .and_then(|node| node.parent_id)
+        .and_then(|parent| nodes.get_node(&parent))
+        .map_or(Vec2::ZERO, |node| node.world_translation);
     propagate_translation(tree, nodes, &bubbled_id, parent_world);
 }
 
@@ -259,7 +263,7 @@ pub fn constrain<
     id: &T::Id,
     parent: Constraint,
 ) {
-    let node = nodes.get_node(id).expect("Id is invalid!");
+    let node = nodes.get_node(id).expect("constrain: Id is invalid!");
 
     let old_constraint = node.constraint;
     let constraint_unchanged = parent == old_constraint;
@@ -314,7 +318,7 @@ pub fn build<
     nodes: &mut N,
     id: &T::Id,
 ) {
-    let node = nodes.get_node(id).expect("Id is invalid!");
+    let node = nodes.get_node(id).expect("build: Id is invalid!");
 
     // Already up-to-date; skip this entire subtree.
     if node.state.is_built() {
@@ -363,7 +367,7 @@ pub fn build_up<
     nodes: &mut N,
     id: &T::Id,
 ) -> Id {
-    let node = nodes.get_node(id).expect("Id is invalid!");
+    let node = nodes.get_node(id).expect("build_up: Id is invalid!");
 
     let constraint = node.constraint;
     let old_size = node.size;
@@ -414,7 +418,9 @@ pub fn propagate_translation<
     id: &T::Id,
     parent_world: Vec2,
 ) {
-    let node = nodes.get_node(id).expect("Id is invalid!");
+    let node = nodes
+        .get_node(id)
+        .expect("propagate_translation: Id is invalid!");
 
     // Already up-to-date; skip this entire subtree.
     if node.state.is_positioned() {