What are the requirements for the 90-day reporting for expats in Thailand?

There are three ways you can do a 90 day report;

in person, mail it in (if your immigration office allows it) and online (again if your immigration office participates in the online program).

The way the 90 day reporting works is

IF you are in the country on the 90th day when your report is due, you need to report. Period end of story. There is NO grace period.

You are confusing the window of opportunity to report in person (15 days before down until 7 days after) with a grace period, it is not.

You cannot think that leaving within the 7 days after your report is due means you are not obligated to do a report.

I have had friends erroneously think that and when they returned to thailand to do their subsequent 90 day report the immigration officer noticed they hadn't filed a previous report and were fined 2000 baht