Anchoring objects left-and-right or top-and-bottom incites FM to stretch the layout to window size in the respective direction. I would say this is intended and expected behaviour. Your observation shows that off-visible part objects are included as well.
Thanks for the hint!
P.S. Positioning the upper left corner of the off-visible object in the body part stops increasing the header and increases the body part instead.
Thank you for your post.
I am able to replicate the issue with FileMaker Pro 14.0.4 under both Mac OS X 10.11.1 and Windows 7. I have forwarded your post and file, along with my findings, to our Development and Testing departments for review. When I receive any feedback, I will let you know.
As you have discovered, the workaround is to not include a bottom anchor on objects outside of the layout boundaries.