let semantic blocks scroll with content (github-bookeditor half)

This needs to go with fe7c4ad00bb29ca5e1fa84d52549cb26e43888af on
Aloha-Editor
Merge branch 'refs/heads/semantics-drag-below-fold-57739880' into testing
Merge branch 'testing' of github.com:oerpub/github-bookeditor into testing