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Steps to reproduce:
1. Create new Unity project using Universal 3D Template
2. In Hierarchy window create Two Pane Split View element (from UI Library)
3. Save New UXML file
4. Create Button element
5. Drag Button element inside Two Pane Split View element
6. Observe the Console window
Actual results: "TwoPaneSplitView needs exactly 2 children." errors are thrown in the Console window when a Button element is dragged inside a Two Pane Split View element in the Hierarchy window
Expected results: No errors are thrown in the Console window when dragging elements inside a Two Pane Split View element in the Hierarchy window
Reproducible with versions: 6000.6.2f1, 6000.7.0b1, 7000.0.0a1
Can’t test with versions: 6000.5.11f1 and lower (feature not available)
Tested on (OS): macOS 26.6.1 (Apple silicon)
TwoPaneSplitView needs exactly 2 children. UnityEngine.UIElements.UIElementsRuntimeUtilityNative:UpdatePanels () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/Native/UIElementsUtility.bindings.cs:22)
TwoPaneSplitView needs exactly 2 children. UnityEngine.UIElements.Panel:Repaint () Unity.UIToolkit.Editor.PanelElement:FrameUpdate () (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Controls/PanelElement/PanelElement+Rendering.cs:170) Unity.UIToolkit.Editor.VisualElementEditingStage:RequestRefresh () (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Stages/VisualElementEditingStage.cs:205) Unity.UIToolkit.Editor.StageCommandHandler:FlushPendingChanges (System.Collections.Generic.IReadOnlyCollection`1<UnityEngine.Object>) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Stages/StageCommandHandler.cs:65) Unity.UIToolkit.Editor.StageCommandHandler:OnCommandGroupEnded (Unity.UIToolkit.Editor.GroupEndedContext&) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Stages/StageCommandHandler.cs:37) Unity.UIToolkit.Editor.CommandSystem:EndCommandGroup () (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/CommandSystem.cs:282) Unity.UIToolkit.Editor.CommandSystem:Execute (Unity.UIToolkit.Editor.Command) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/CommandSystem.cs:208) Unity.UIToolkit.Editor.UICommandQueue:Execute (Unity.UIToolkit.Editor.Command) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/UICommandQueue.cs:67) Unity.UIToolkit.Editor.ReparentElementsCommand:Execute (object,UnityEngine.UIElements.VisualElementAsset,int,UnityEngine.UIElements.VisualElementAsset[]) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/ReparentElementsCommand.cs:27) Unity.UIToolkit.Editor.VisualElementNodeHandler:DoPerformVisualElementsDrop (UnityEngine.UIElements.VisualElement,int,System.Collections.Generic.List`1<UnityEngine.UIElements.VisualElement>) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Hierarchy/VisualElementNodeHandler+DragAndDrop.cs:325) Unity.UIToolkit.Editor.VisualElementNodeHandler:HandleVisualElementBeingDragged (Unity.Hierarchy.HierarchyViewDragAndDropHandlingData&,UnityEngine.UIElements.VisualElement,int,bool) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Hierarchy/VisualElementNodeHandler+DragAndDrop.cs:212) Unity.UIToolkit.Editor.VisualElementNodeHandler:HandleDrop (Unity.Hierarchy.HierarchyViewDragAndDropHandlingData modreq(System.Runtime.InteropServices.InAttribute)&,bool) (at /Users/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Hierarchy/VisualElementNodeHandler+DragAndDrop.cs:61)
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