User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.101 Safari/537.36 Steps to reproduce: 1. Be using the UX branch of Firefox 27 (http://hg.mozilla.org/projects/ux) on Mac OS X 10.9 (Mavericks) 2. Run a bootstrapped addon whose main panel scrolls vertically (ideally with some way to view stderr) 3. Scroll vertically Actual results: Quartz 2D sends errors to stdout similar to this: "firefox-bin[17318] <Error>: CGContextTranslateCTM: invalid context 0x0. This is a serious error. This application, or a library it uses, is using an invalid context and is thereby contributing to an overall degradation of system stability and reliability. This notice is a courtesy: please fix this problem. It will become a fatal error in an upcoming update." I'm not familiar with Quartz 2D, but this looks to be due to a null graphics context somewhere. No matter where you send stderr, whether to a terminal emulator, /dev/null, or a text file, the blocking output created by Quartz 2D itself will cause scrolling to stutter. Expected results: Quartz 2D shouldn't be printing errors, and scrolling shouldn't stutter.

Bugzilla caught my user agent string from Chrome because I was busy using Firefox to demonstrate this bug - here's the user agent of the browser in question: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:27.0) Gecko/20100101 Firefox/27.0

Can you attach a copy of the add-on that you refer to in step 2 of your STR?

These warnings also show up when typing in e.g. the browser console and its scrollable autocomplete popup shows up.

> invalid context 0x0 I've seen errors like this from Firefox (and other programs), in ones and twos, for years, on different versions of OS X. I've never thought much about them, because we've never been able to reproduce them and they didn't appear to be harmful in themselves. However, if we *are* now able to reproduce them reliably enough (with your extension), we may be able to do something about them. > the blocking output created by Quartz 2D itself will cause scrolling to stutter I suspect this only true if that output is being displayed somewhere -- say in Terminal or the Console app.

Have you tested on other versions of OS X?

The patch in bug 951443 may fix this bug.