I've only tested this on Windows 10, so am not sure it applies to prior versions of Windows.
You'll need to rewrite some code if you depend on sometimes having a script start with turning User Abort ON and eventually call a Document Window that exposes the OS Close (X) button on the called window's titlebar, while pausing the calling script with an Indefinite Pause. This is how we used to create modal windows before FM finally provided modal windows as a built-in feature and also allowed us to remove titlebar buttons, like the OS Close (X) Button. I still to do this, instead of using an actual Dialog (modal) Window, if I want to call another window from the called modal window (e.g., to do offscreen processing when the modal window is active). That's because calling a window from a Dialog Window is not permitted (FM will allow you to script that, but it will be ignored during runtime!). This has been true since FM first provided us Dialog Windows.
So, if you do create such a modal window by using a Document Window called after User Abort is ON and Pause Indefinitely, you'll now find, on Windows 10, the user can close that modal window by clicking the OS Close (X) Button (if present). This is not true when doing this with FileMaker 15. In the latter case, when the user clicked the OS Close (X) Button, that keypress is ignored. This is new behavior on Windows, using FileMaker 16. The recoding you'll need to do is remove the OS Close (X) Button from the called Document Window in each such case. This behavior change is not present when running FileMaker 16 on the Mac.