In Safari version 12.1, Apple changed the behavior of password AutoFill. According to Apple, Safari now "Streamlines website login when filling credentials with Password AutoFill". This means that when you AutoFill a login form, Safari not only fills in the password and other login fields but also automatically submits the login form. Unfortunately, this new behavior can cause problems. There are some excellent articles by Glenn Fleishman and Michael Tsai discussing the problems with Safari login autosubmit in more detail. Fortunately, StopTheMadness can now stop Safari from autosubmitting login forms!
StopTheMadness has a new feature called "Stop autosubmit of autofilled forms". This feature was introduced in StopTheMadness version 8.0 and refined in versions 8.1 and 8.2. By default, "Stop autosubmit of autofilled forms" is disabled, so if you want this feature you need to enable it manually in the StopTheMadness Website Options. You can selectively enable the feature for individual sites, or you can enable it in "Default options for all websites". There are instructions for how to add website options at StopTheMadness for Safari Support.
When "Stop autosubmit of autofilled forms" is enabled in StopTheMadness, Safari will no longer automatically submit forms. Instead, StopTheMadness will display a confirmation that asks whether you want to submit the form or cancel. If you cancel, then you can make changes to the AutoFilled form and submit manually. Note that when the confirmation is displayed, you can press the return key to submit or press the escape key to cancel if you want to avoiding clicking a button.
The only catch to this new feature is that StopTheMadness has no way to distinguish between Safari automatically submitting a login form and you manually submitting a login form. When Safari AutoFills a form, Safari generates fake keypresses and clicks that appear to be just like real keypresses and clicks. Thus, the only solution to this problem was to require confirmation for login form submissions, whether they're submitted automatically by Safari or submitted manually by you. The only way to get the new feature to work was to require confirmation, even though confirmation requests can be a little annoying. That's why "Stop autosubmit of autofilled forms" is disabled by default.
Here's how it works: when Safari autosubmits a login form, StopTheMadness asks for confirmation. If you press return or click OK, the login form gets submitted. If you press escape or click Cancel, the login form does not get submitted, but the form is still AutoFilled. After you cancel, you can make whatever changes you need and then submit the form manually. StopTheMadness only requires confirmation on the first submission, so you won't see a confirmation request again on resubmission.