Distinguish between default zero and intentional zero sort order for new documents (#17517)

* Distinguish between default value and initial zero

* Update special value comment documentation

* Redid solution with dirty/new entity tracking

* rework copy branch sortorder fix

* Change == false to is false

---------

Co-authored-by: Mole <nikolajlauridsen@protonmail.ch>

src/Umbraco.Core/Services/ContentService.cs

@@ -2760,6 +2760,9 @@ public class ContentService : RepositoryService, IContentService
                         descendantCopy.CreatorId = userId;
                         descendantCopy.WriterId = userId;
 
+                        // since the repository relies on the dirty state to figure out whether it needs to update the sort order, we mark it dirty here
+                        descendantCopy.SortOrder = descendantCopy.SortOrder;
+
                         // save and flush (see above)
                         _documentRepository.Save(descendantCopy);
 

src/Umbraco.Infrastructure/Persistence/Repositories/Implement/DocumentRepository.cs

@@ -900,9 +900,10 @@ public class DocumentRepository : ContentRepositoryBase<int, IContent, DocumentR
         NodeDto parent = GetParentNodeDto(entity.ParentId);
         var level = parent.Level + 1;
 
-        var sortOrderExists = SortorderExists(entity.ParentId, entity.SortOrder);
+        var calculateSortOrder = (entity is { HasIdentity: false, SortOrder: 0 } && entity.IsPropertyDirty(nameof(entity.SortOrder)) is false) // SortOrder was not updated from it's default value
+                                 || SortorderExists(entity.ParentId, entity.SortOrder);
         // if the sortorder of the entity already exists get a new one, else use the sortOrder of the entity
-        var sortOrder = sortOrderExists ? GetNewChildSortOrder(entity.ParentId, 0) : entity.SortOrder;
+        var sortOrder = calculateSortOrder ? GetNewChildSortOrder(entity.ParentId, 0) : entity.SortOrder;
 
         // persist the node dto
         NodeDto nodeDto = dto.ContentDto.NodeDto;