Repaints are not such a big problem, but they are a distraction. A repaint affects the appearance of some rectangular area within the page, but does not cause a reflow of the entire page. A repaint can still derail the user’s train of thought, but not as severely as a reflow does.
Reflow is sometimes called reload or relayout. Repaint is sometimes called redraw. A reflow will cause a repaint, but a repaint will not cause a reflow.
Understanding the page rendering process is essential for anyone who wants to get a handle on client-side performance issues. Simply applying tips without understanding how they work can be a fruitless endeavour.
The following reference section contains a few best-of-the-best articles that help us understand page rendering (especially reflow). These articles are so well written that I cannot present a better explanation. The most useful thing I can do is point you to them, which I do now.
by Stoyan Stefanov. Published 2009.12.17 by Stoyan Stefanov. Accessed 2012.04.11.
This is the best explanation of rendering, repaints, reflows, redraws, restyles, relayouts, resizing, reformatting, requesting style info, and whatever other R words Stoyan can come up with. It provides a detailed explanation of the rendering process, including the difference between the DOM tree and the render tree, and the difference between a reflow and a repaint in terms of its impact on the render tree. It lists and demonstrates reflow/repaint triggers (and cautions us not to be trigger-happy).
After acknowledging that modern browsers are smart enough to batch reflows, it advises that the programmer should batch reflows himself for even better performance and warns us that querying style and layout information can eliminate the benefits of batching.
by Juriy Zaytsev. Published 2012.01.04 by Juriy Zaytsev. Accessed 2012.04.11.
This article recounts the author’s experiences while optimizing a one-page web application that uses lots and lots of CSS3. It details his performance results, including measurements, and ends with a few tips regarding CSS’s impact on reflow. Some of these tips are new. Did you know that page zoom affects CSS performance?
by Nicole Sullivan. Published 2009.03.27 by Nicole Sullivan. Accessed 2012.04.11.
This article distinguishes between reflows and repaints, lists some of the things that cause a reflow, and describes six tips for minimizing the number or impact of reflows. The See It In Action section provides links to videos that help us visualize the reflows associated with loading Google’s, Mozilla’s, and Wikipedia’s web pages.
by Nicholas C. Zakas. Published 2009.06.13 by Nicholas C. Zakas. Accessed 2012.04.11.
Pages 67-95 of this slideshow demonstrate how to avoid reflows and optimize DOM-tree interactions. The code samples are especially helpful.
This is a quick summary of the tips included in the above articles. For a full discussion, please read the articles. I’m begging you … please read the articles. And if you’re going to skimp, read the first ones first.
And one additional tip from yours truly: Invest in the slowest, least advanced computer available on the shelves. Use it for testing your webapps.