Due to historical reasons - some important cells are rounded (all aspect ratio cells actually).
Which gives not-precise calculations of deviation (not the same as on the real device), as so thresholds in the spreadsheet may not fit real device's needs!
I can't believe I was so stupid...
https://developers.google.com/web/updates/2016/12/url-bar-resizing
This may lead to change in impl (e.g. check for equality between HTML element clientHeight and window.innerHeight. Other browsers are still on older Chrome versions, but if they will upgrade - they most probably will start to behave in similar manner.
On each launch the script should recreate all feature files (file name should correspond to each sheet in the spreadsheet). Diff management will then be handled by Git.