Safari 9.1 truncates alert/confirm/prompt dialogs?

Question

(Inherited webapp, don't blame me. I'll likely redesign around this in the future.)

Customers complained about "missing prompts" when using latest Safari on OSX. I compared it to a much older version, and saw that the prompts used to be displayed correctly.

Upon further inspection, I realized that the prompt was now being cut off, hidden in a scroll pane. And since Apple's design calls for scrollbars to be initially invisible, the users had not indication that they needed to scroll.

I could not find any reference to this change. Upon reading the latest changelog for Safari (linked below), I noticed that other changes had been implemented to combat malicious popup behavior.

Could this be an undocumented "enhancement"? Besides using jQuery Dialogs (with callbacks) or some equivalent, is there any way to show the entire message? Whitelist the site or trusted sites in Safari?

https://developer.apple.com/library/mac/releasenotes/General/WhatsNewInSafari/Articles/Safari_9_1.html#//apple_ref/doc/uid/TP40014305-CH10-SW11

alert('this\nis\na\ntest\nthis\nis\na\ntest')