It's not 7 days of non-use, it's seven days of application use without visiting the site.
Safari is one application, the homescreen app is a separate application. Presumably, all the alt browsers or WebView apps are separate applications as well.
Since you can't use a homescreen app without visiting the site, the 7 days of not visiting the site can't happen.
What if you visit a link within the home screen app that takes you to another domain? Presumably if you kept using it with ever returning to the original domain the clock would be ticking.
For "regular websites" (visited through Safari) it's 7 days where you use Safari, but don't visit the site. So if you go on vacation for a month and don't touch your computer, or if you switch completely to using Firefox for a month, localStorage will remain untouched.
I fail to see why you need to see a mention of "regular websites". The comment clarifies the situation of what occurs if a user goes on vacation or switches to another browser: nothing will be deleted, as Safari is not being used.
It’s a requirement that some time within the next seven days of app usage, the user interacts with the web app.
This might cause trouble if the web app is simply a list of timers which the user interacts with passively (map of earth showing day/night zones), but if there is any interaction at all the timer resets.
I'm not getting the semantics clear but wonder whether having the icon on the homescreen counts as "visiting" or whether suspending the app first day and reopening it the next day counts app-subjectively as "continuing day one" or "reopening immediately"
From their description, nothing. Seven days of use without visiting triggers deletion. Failing to satisfy either of those conditions (either by disuse of Safari, or by visiting the site within seven days) doesn't.
It's not 7 days of non-use, it's seven days of application use without visiting the site.
Safari is one application, the homescreen app is a separate application. Presumably, all the alt browsers or WebView apps are separate applications as well.
Since you can't use a homescreen app without visiting the site, the 7 days of not visiting the site can't happen.