[Performance] Optimize Paint and Composite for the website

"Paint" is one of the most preference killer, it can easily cost more than 60fps, and once you trigger "Paint" it always trigger "Composite" as well.

First  of all, let's see how to use Chrome devtools to help us identifiy the Paint problem.

 a. turn on rendering:

Then check the "Paint flasing" option.

Then use this demo  site, keep "Paint flashing" open, and you should see when you scrolling the page, the whole page is repainting (mark in green overlay).

Of course the whole page is re-painting is not good sign for the performance. 

Record the timeline for the demo page and scroll a bit, then check the Paint Profilter:

From here you can pretty much see all the commands it runs and each step what is painted to the page.

Composite:

To avoid too much painting, we can put different compoment in different layouts.

For example the side-nav compoment, to prevent the whole page re-paint again and again, we can put side nav in a different layout, so that the main page won't re-paint.

Prompt element into a layout:

Code that works:

will-change: transform;
transform: translateZ(0); // hack for some browser not support will-change

DEMO site, you can see it move really slow, but if you add the css to .box, the performance improve a lot.

Notice that, only using layer promption when you are changing the position, opacity stuff, if you want to change text in the element, layer promption doesn't make any sence

Now let's say how to figure out how many layers we have in the site.

We need again our chrome devtools to help.

Select "Layout", and nav to the demo site.

Zoom in the page, until you are able to scroll the page. 

Then see the dev tool:

In the layers section, you able to see how many layers you have in the page. And it tell you the reason why it is a composition layer, in the image, it says that "Composition due to association with an element with a css 3D transform".

If you check the css code for "#color-block":

#color-block {
    width: 300px;
    height: 300px;
    background: red;
    transform: translateZ(0); // this is the key
    position: relative;
    z-index: 0;
}

And if we select "totes-promoted" block, we can see that "Composition due to association with an element overlapping other composited elements".

So be careful that when you prompt one element into a layer, the overlapping elements will be also prompted as well.

This may cause memory problem.