Avoiding endless loop with validateField + JS alert()

I'm very rusty with my client script, but I do recall running into this same problem years ago, hopefully someone here has a solution.

Requirement: Using client SuiteScript restrict edits to the quantity column on sales orders and display an alert to the user. No Workflow, no custom forms, no UE script to make columns disabled - must be client side SS. This isn't my real use case, but it accurately represents my technical needs.

Problem: NetSuite fires the validateField event whenever a field loses focus. The validateField handler is the natural choice to reject a user edit (by returning false). If my quantity field receives focus, then the user clicks out of the browser window the validate event is raised, my client code runs and shows the alert. When the user clicks the alert OK button, it triggers the validateField event again and so begins the endless loop.