What's with the checkbox saying "prevent the page from showing any more dialog boxes"?

This is not a feature of FairSplit, but of your browser.

In the past, misbehaving websites were able to create very long sequences of dialog boxes to annoy users. This is why now browsers provide this feature -- on the second dialog box created by a website in a short period, the browser allows you to turn the dialog boxes off altogether.

There is no way for any web application to prevent the browser from showing the checkbox since it is a "security feature" of the browser, which never knows if the page is behaving or not.

In FairSplit you should never check the box because the dialog is usually a confirmation before the application actually deletes data. The application won't remove anything without a confirmation from you.

If you have already enabled the checkbox, you might have temporarily locked yourself out of some functionality, for instance, deleting images.

You should be able to cause the browser to forget the choice previously made on the checkbox if you close the tab, close the browser, and reopen everything.