I can confirm that this change works on my site using Mac OS High Sierra 10.13.1 and Safari 11.0.1 as well as Google Chrome 62.0.3202.94.

My wiki isn't failing. It's working fine. This error was displayed on the home page, however, so it was problematic. I disabled warnings to get rid of it. I'll be happy to provide any information that is helpful to assist in tracking it down. It's merely a warning, not a fatal error.

The directory is read and writeable by the webserver. Here's the relevant php code:

227         // compile locking
228         if (!$_template->source->handler->recompiled) {
229             if ($saved_timestamp = $_template->compiled->getTimeStamp()) {
230                 touch($_template->compiled->filepath);
231             }
232         }

Frankly, I'd be inclined to tell the user to upgrade their browser. :)

So what is supported? And what would happen in older versions of IE? Would it crater? Or just not work?

I found out that the code is in jquery.WikiEditor.publish.js.
This is the relevant section:

Where do I find the code? I'll take a look, but I'm not a programmer, so....

Has this been closed? Or is it still being worked on?