As many of you know, Squarespace is my platform of choice for web hosting and I typically use Safari as my default browser on all my Macs. Apple has continually enhanced Safari, and while they have a Safari Dev channel, sometimes other developers cannot always keep up, or a change is not clearly documented and this causes problems, hence the current issue between Safari and Squarespace.
Here's the short answer. Don't use Safari 9.x to work on your Squarespace site for the time being. If you are a Windows user, I cannot fairly direct you, since I only use Windows as a test bed. If you use a Mac, I have proven that the current release of Google Chrome is completely stable for working with Squarespace.
I was working on a lengthy article and Safari blew up and I lost most everything. I had become sloppy in saving, partly because I get used to Autosave and partly because when you hit Save in Squarespace it also closes your document which is a bad UI design in my opinion.
After three strikes, I contacted Squarespace support who as always responded in a very timely manner and they advised that they were aware of issues with the current Safari and "were working on it" That was three weeks ago, and in the absence of any notifications or updates from Squarespace, I did another Safari test with this very post. Suffice to say I had the opportunity to experience another state of KABOOM!.
I am sure that Squarespace will figure this out, what I don't know, and neither do they, is when. So to save yourself frustration, migraine and the loud use of creative metaphor, don't use Safari to work on your Squarespace site until further notice.