If user tries to close a page while script is running, there should be a notification

[bugy]

Currently, if user closes a page, it just closes silently and running script is stopped.

This can be not very obvious for a user (or user can close a page unintentionally), so some confirmation (probably) should be asked.

[bugy]

Unfortunately modern browsers doesn't allow to have custom windows close confirmation. So a standard dialog is shown (e.g. "there are some unsaved changed" in chrome).