This was originally posted by Robert O’Callahan in the Mozilla web-tech blog. It’s an interesting feature in Firefox 3.5 and is worth repeating here as part of our 35 days effort.

In addition, Thomas Robinson has created a very handy bookmarklet for debugging painting on a page you’ve loaded in the browser.

Due to popular demand, we’ve created a very experimental API for Firefox 3.5 to fire an event every time content is repainted. The event is is called MozAfterPaint and is fired at the document, bubbling up to the window. The event offers two attributes, clientRects and boundingClientRect, which tell you what was repainted, using the same objects and coordinate system as the getClientRects and getBoundingClientRect methods.

This is very useful for Firefox extensions and other “chrome” code that might be using the canvas.drawWindow method to capture the contents of windows. It might also be useful for tools like Firebug. But it’s also potentially useful for regular content, for example if you want to add some lightweight JS instrumentation to a page to measure what gets painted by Firefox, and when.

Caveats:

• This is Gecko-only. Do not use this for actual functionality on public Web pages – although I’m not sure why anyone would, so I don’t currently see this as a candidate for standardization.
• For security reasons, regular Web content is only notified of repainting that occurred in its own document – repainting caused by IFRAMEs is not reported to untrusted listeners attached to the IFRAME’s ancestors. (Listeners added by “trusted” content such as Firefox chrome are notified of all repaints to the window, however.)
• Currently the event might fire before the actual repainting happens. This shouldn’t matter much, and we’ll fix that at some point.
• If your event handler does anything that triggers repainting, such as changing the style of an element, you will probably trigger an infinite loop. The browser should stay responsive but your machine will contribute to global warming.
• Repainting of areas scrolled outside the viewport is reported, but repainting of areas scrolled outside overflow:auto elements and the like is not reported.
• Repainting in windowed plugins (i.e. most plugins in Windows and GTK) is not reported.