Paint Flashing is now in the official Release channel starting in Firefox 11. I posted a new add-on that let’s you enable Paint Flashing with one click: https://addons.mozilla.org/en-US/firefox/addon/toggle-paint-flashing/
This tool is useful for quickly inspecting how Firefox renders your web pages as it visually indicates how much screen area is being painted after the page layout is computed.The source code for the add-on is also posted.
As promised, I’m going to call out Firefox Layout & Rendering code changes that my team has been working on. This bug fix from Bas Schouten (as reviewed by Robert O’Callahan) was one of those light-bulb code changes that have really advanced my understanding of the Gecko Layout Engine’s graphics code:
This check-in implements “Paint Flashing”, a diagnostic tool that shows when the browser is invalidating (or repainting) a screen region that has changed. It’s very useful for diagnosing redundant (painting too often) or greedy (painting too large an area) invalidation. Both kinds of bugs are notoriously hard to find.
Paint Flashing will paint a random color (at 20% opacity) to indicate the “damaged” area of the screen that the browser is repainting. You can enable Paint Flashing by setting this preference to true: