FDetailGroup::AddGroup does not correctly apply the default expansion state for DetailGroups

There is a bug affecting DetailGroups in the engine. When adding child DetailGroups to a parent DetailGroup, the expansion state does not behave as expected and ignores the flags intended to control it.

This appears to be an issue related to the order of operations: although the underlying data becomes valid later in the process, the Slate widgets are not constructed correctly, and the DetailGroup expansion state is ignored. The user also observed that when calling AddGroup, the OwnerTreeNode is invalid, which likely prevents the expansion state from being applied.

1- Obtain an IDetailGroup to edit from the editor. In this reproduction case, a NomadTabSpawner was created and a DetailsView was added to it. An IDetailCustomization was then registered to allow adding and editing DetailGroups.

2- Once access to the DetailGroups is available, create multiple groups in a hierarchy. For example:

// Parent Group

IDetailGroup& ParentGroup = Cat.AddGroup(

"ParentGroup1",

FText::FromString("Parent Group"),

/bStartExpanded/ true

);

//Int variable

ParentGroup.AddPropertyRow(IntHandle);

// Child Group

IDetailGroup& ChildGroup = ParentGroup.AddGroup(

"ChildGroup1",

FText::FromString("Child Group"),

/bInStartExpanded/ true

);

//Float variable

ChildGroup.AddPropertyRow(FloatHandle);

3- After creating the child group, attempt to expand it programmatically:

ChildGroup.ToggleExpansion(true);

4- Open the edited widget.

Observe that the DetailGroups are not expanded, despite the expansion being enabled both via the AddGroup function and the ToggleExpansion method.