You definitely should be able to stop the autosaving, and use the settings menu drop-downs in general. I’m having trouble recreating the issue myself, so it’s gonna be hard to find. As far as anything you can do, I appreciate the offer. If you can give me the version number of Chrome you are running, that would be great. You can find it by typing chrome://version/ into the URL bar of the relevant browser.
The autosave does use its own dedicated slot separate from manual saves, and there is only one slot. Saved games are compressed so should be quite small, but you should still be able to shut off autosaves, so this is still an issue either way and not ideal or intended.
I will try to find a solution to the bug, worst case scenario is to change the way the settings menu is implemented to totally avoid the snap-back effect in the first place, which is unsightly even when it is working correctly. However, doing that will take time.