官方文件是这么说的：

If you are in the United States and wish to travel abroad, you do not need to aaply for advance parole if both conditions described below are met:

1. You are an H-1, temporary worker, or H-4, spouse or child of an H-1.

2. A form I-485, Application to Register Permanent Residence or Adjust Status, was filed on your behalf and is pending with the USCIS.

However, upon returning to the United States, you must present your valid H nonimmigrant visa and continue to remain eligible for that status.

对加拿大公民来说，关键是H nonimmigrant visa。但是加拿大公民又不需要visa，如果你去使馆，我甚至怀疑给不给你签visa，这似乎是一个灰色区域。