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++ // Copyright (C) 2026 The Qt Company Ltd.
++ // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
++
++ // Takes the scene away from the world while it is walking its bodies to sync
++ // them. A handler on the position a sync writes moves the world to another
++ // scene, which deregisters every body at once, so each body the walk has not
++ // reached yet is left with nothing to sync. Every body carries the handler, so
++ // whichever the walk reaches first does the moving and the ones behind it are
++ // the ones left behind. Syncing them crashed.
++ //
++ // Nothing is deleted here, and that is the point: a scene change leaves a
++ // backend without a frontend just as a delete does, and it lands while the walk
++ // is still on the stack, rather than whenever the object a delete was asked for
++ // is deferred to.
++ //
++ // Reaching the end of both of the frame's walks is the assertion, asked for as
++ // two more finished frames after the move.
++ //
++ // No rendering is needed for any of this, so scene is a plain Node rather than
++ // a View3D: debug drawing below builds its models against that node and nothing
++ // asks to see them.
++
++ import QtQuick
++ import QtTest
++ import QtQuick3D
++ import QtQuick3D.Physics
++ import QtQuick3D.Physics.TestUtils
++
++ Item {
++ id: root
++ width: 64
++ height: 64
++
++ // The frame the world was moved on, set by whichever body the walk reached
++ // first.
++ property int frameAtMove: -1
++
++ PhysicsWorld {
++ id: world
++ gravity: Qt.vector3d(0, -981, 0)
++ running: true
++ // On, so the frame that loses its scene mid-walk goes on to the second
++ // walk it makes over the same backends, the one debug drawing does.
++ forceDebugDraw: true
++ typicalLength: 100
++ typicalSpeed: 1000
++ minimumTimestep: 15
++ maximumTimestep: 15
++ scene: fallingScene
++
++ property int frameCount: 0
++ onFrameDone: world.frameCount++
++ }
++
++ Node {
++ id: fallingScene
++
++ // Falling freely, so every sync has a new position to write and no body
++ // ever falls asleep and stops being written to.
++ Repeater3D {
++ model: 3
++ DynamicRigidBody {
++ required property int index
++ position: Qt.vector3d(index * 120 - 120, 200, 0)
++ collisionShapes: BoxShape {}
++ // Held off until the world has frames behind it, since setting
++ // the position above emits this too, and moving the world from
++ // there would move it before it ever walked anything.
++ onPositionChanged: {
++ if (world.frameCount > 5 && world.scene === fallingScene) {
++ root.frameAtMove = world.frameCount;
++ world.scene = elsewhere;
++ }
++ }
++ }
++ }
++ }
++
++ // What the world is moved to. It holds no bodies, so the move leaves every
++ // body without a world rather than handing it to a new one.
++ Node {
++ id: elsewhere
++ }
++
++ PhysicsTestCase {
++ name: "moving the world while it syncs its bodies"
++ goalReached: root.frameAtMove >= 0 && world.frameCount > root.frameAtMove + 1
++ }
++ }