@b3m2a1 & anyone else experienced with front-end stuff: I have an issue with cell grouping (example notebook: https://www.dropbox.com/s/afwiscrbp7efkhs/CellGrouping.nb?dl=1 - no wolfram cloud link since the cloud breaks the notebook...):
* The two cells are not grouped, and the file is immediately marked as having unsaved changes.
* Editing anything causes the two cells to be grouped and the top one to become a proper opener.
* If you "fix" the file and save it again, it will be broken again when reopened