Quick update - I found that a test that looks like this:

https://gist.github.com/smspillaz/8138673

When only using G_FILE_CREATE_REPLACE_DESTINATION and not g_file_delete before it fails because while the first part of the file is overwritten, the existing contents may remain afterwards.

I suspect that's some breakage on the part of a git version of GLib I'm using, so maybe we should try to fix this upstream also.