Xdebug registers opcode handlers for code coverage, exit, and tracing. These currently return the ZEND_USER_OPCODE_DISPATCH value which means that the Zend engine users its internal implementation to further handle that opcode. If however another extension (such as https://github.com/nikic/scalar_objects) has also overridden the opcode, its handler will not be called.

Make sure to remember already set handlers, and then call these if they're not NULL, and otherwise continue returning ZEND_USER_OPCODE_DISPATCH.