[docs] Fix Recalculate style regression

[oliviertassinari]

I was browsing the docs and felt that it was noticeably slower than it used to be. So, out of curiosity, I opened my dev tool. As it turned out, this is a regression from #3493. We introduced a complex CSS selector that breaks the CSS performance on the page:

body:has(.RootLayout) ::selection {

Two ways to reproduce the problem:

Open Select

Screen.Recording.2025-12-13.at.01.45.49.mov

• HEAD: 99ms

• PR: 26ms

Page transition

Screen.Recording.2025-12-13.at.01.49.40.mov

• HEAD: 340ms

• PR: 54ms

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A must-watch on the topic: https://www.youtube.com/watch?v=nWcexTnvIKI. In our case:

My small contribution to v1 🎁. There is still stuff that feels slow, but one step at a time 😁

[pkg-pr-new]

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@3531
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@3531
  

commit: 1e79f9e

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	0B(0.00%)	0B(0.00%)

Details of bundle changes

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[atomiks]

Nice find! This is quite noticeable, even on a fast machine.

I was actually wondering why interactions felt "slightly" slower than before when browsing around earlier. On 4x-6x slowdown, it's pretty terrible and very noticeable 😬

[atomiks]

I've deployed this change as it felt urgent, will be live shortly: https://base-ui.com

[oliviertassinari]

There is still stuff that feels slow, but one step at a time

To illustrate my point, I made:

• [docs] Fix layout trashing scrollable #3535
• [select] Forced reflows / Layout trashing #3533
• [scroll area] Forced reflows / Layout trashing #3534

But I can't push this further, no time 😁