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. The fix should be do the same as ToggleButtonGroup, i.e. silently not add the child if it's not valid. Or perhaps just auto-move it into the first element as though it was the contentContainer. and change the errors into warnings

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)