Need "discard changes" warning before exiting custom field edit

[#1]

When editing a custom field from the "Fields" tab of Custom Forms, admins can easily accidentally click away from the screen and all work is lost.  It has happened to me several times, even when I was conscious of the issue and being extra-careful (just a stray click will do it).  There should be a mechanism in place that prompts the user to confirm they want to discard unsaved changes.