Scrolling is so smooth 🧈

The headers now need a background-color because they float in front of the content. Not sure how to resolve this in a generic way. This change would probably require a change in our users' codebase.

Same as for pinned column: #5979. It's probably OK. At least, as a developer, I don't mind the pain to customize this, in exchange of the smoother UX.

Isn't this much better?

Before: https://deploy-preview-11650--material-ui-x.netlify.app/x/react-data-grid/#commercial-versions

After: https://deploy-preview-10059--material-ui-x.netlify.app/x/react-data-grid/#commercial-versions

@oliviertassinari, try disabling overscroll-behavior, and will look and feel even more performant on iOS (as I pointed out here: #11230 (comment)). You wont have all the rows disappearing for a brief moment when you accidentally overscroll on the X-axis while actually just trying to scroll down ;)

E.g. this won't happen:

@romgrk I found a regression, this PR broke this demo: https://next.mui.com/x/react-data-grid/row-updates/#lazy-loading

You wont have all the rows disappearing for a brief moment when you accidentally overscroll on the X-axis while actually just trying to scroll down ;)

@lauri865 I would expect #11230 to solve this. It also sounds like a performance issue, I don't get less white screen on AG Grid.

For overscroll-behavior, I wouldn't touch this. It both sounds the expected default behavior for users on iOS and a requirement to be able to propagate scroll up in the parent chain.

Opened the issue linked above.

@romgrk In terms of scrolling performance, I have noticed a few things on this PR.

First, the baseline, ranked by how much time it takes to render the next rows, the difference is clear on my laptop (in production and also more noticeable in dev mode):

• 🥇 a. AG Grid: no white screens
• 🥈 b. Before: a little bit of white screen, about 8% of the viewport
• 🥉 c. After: more white screen, about 20% of the viewport

Now, when we look at the actual profile for 1,000 ms of scroll, we see a much different story:

• AG Grid: no white screens, overscan of 10 rows

• Before: a little bit of white screen, about 8% of the viewport, overscan of 3 rows

• After: more white screen, about 20% of the viewport, overscan of 3 rows

We were 1st Before, and clearly, I think the most important metric to judge performance is idle time. However, because we are not using a high enough overscan, end-users would feel a white area. It seems clear to me that 3 is not enough https://mui.com/x/api/data-grid/data-grid/#DataGrid-prop-rowBuffer. Increasing it would be a perfect quick win.

Another opportunity I see is how we re-render emotion components at each render, e.g. ScrollbarFiller. How about we memoize or move the style outside of the re-render flow during scroll?

Now, the problem. Vertical scrolling is a more frequent use case than Horizontal scrolling, so should be prioritized in terms of UX if we can only have one. So I think for this PR to benefit end-users, we need to get to the bottom of this regression. I suspect we can fix it. If we can't, and it's because of a core tradeoff of the sticky position, then I think it would be better to stay on the previous side of the tradeoff (not using it). We are no longer ahead of AG Grid, more like a tie now (once we increase the overscan and save Emotion rendering cost).

So, I drilled deeper, there is a x2 increase in "Rendering" time in After compared to Before. This seems to come from two things:

• We spend a lot more time in the Layerize phase (https://developer.chrome.com/docs/chromium/renderingng-architecture).

As I can see it, it's because of this style:

Once this style removed, the computation bottleneck seems to then move to recalculateStyles, this is very odd to me:

More in context:

I don't know why. I ran out of time I can allocate to this 😄

Yes I've noticed the performance changes, I merged anyway because I wanted to get the breaking changes in. I kinda expected the layout to be more expensive for the browser due to that transform style as it's on each cell now instead of on a wrapping container, but I think it will be fairly easy to add a wrapping container for the cells and apply the style once. That in conjunction with the CSS contain: strict style should make it much more easy for the browser to layout stuff. The CSS containment styles are fairly easy to use for us because we know our dimensions ahead of time with single pixel precision thanks to the changes in the dimensions logic. We compute dimensions once and those dimensions usually don't change much, even during scrolling.

I also initially used CSS variables a lot in this PR, often in dynamic ways (e.g. calc(10px + var(--xxxx)), but ended removing them in a few places and might end up removing a lot more of them, because they also increase the layout time.

Regarding emotion, that library is absolutely atrocious for runtime performance and the sooner we can get rid of it the happier I will be. I have been trying to find a way to get emotion to do something like this:

const className = css` color: red `

instead of having to use their styled-components, but I don't know the library and our system style wrappers much so I also wasn't able to complete this during this PR. Discussing how we do styling internally on the datagrid is on my roadmap.

Lastly regarding the row overscan, I've already opened #11344 to discuss that aspect, read there for more details.

@romgrk Alright, let's continue to explore this then. Overall, it's not yet clear that we should sell it as a performance improvement in the v7 release blog post, but it feels like we are close 😁

I have been trying to find a way to get emotion to do something like this:

A nested CSS selector from the parent does something close, as long as that parent doesn't rerender. We moved many styles up to the grid root in the past because of this.

CSS variables

I don't know if they increase layout time, I wouldn't expect them to.
However, updating a CSS variable 120 times per second is slow as the browser needs to make the value propagate down all the children, not just the element itself (unless maybe if we use a custom CSS property with inheritance disabled).

A nested CSS selector from the parent does something close, as long as that parent doesn't rerender. We moved many styles up to the grid root in the past because of this.

That's terrible. We don't get the CSS-in-JS advantage of colocation of styles & logic, and we don't get the CSS advantage of native (clean) CSS syntax. I've been observing GridRootStyles, I also don't like that the massive styles object is recomputed, and it's recomputed quite often during normal usage of the grid despite being near the root :|

I've been watching the zero-runtime solution in the hopes that it solves all our style issues, in the meantime if there's any solution that would help with injecting CSS simply & directly, I'd definitely adopt that.

Extracted vertical scrolling performance to a separate issue: #11866

I've opened #11924 for the performance issues.

Last fix for the issues above opened in #12019. CSS variables were the culprit.