Organizer dialog is split
Not version-specific
Tekla Structures
Environment
Not environment-specific
If an Organizer dialog, like Categories or Object Browser are split in separated windows, you can reset the dialog positioning by deleting the AvalonDockConfig.xml file, then reopening Organizer.
C:\Users\<user>\AppData\Local\Trimble\Tekla Structures\ProjectOrganizer\AvalonDockConfig.xml