Chromium: excessive layout time with many math previews due to unused CSS anchor

What’s broken?

Rendering 500 official inline math previews in Chromium spends about 17.6 seconds in layout, including with editable={false}. Every .bn-preview-container receives anchor-name: --bn-source-popup-anchor in packages/core/src/editor/Block.css, although the current popup positioning does not consume this anchor.

Removing that declaration reduces layout to about 0.43 seconds with the same content and DOM.

What did you expect to happen?

Closed source previews should not incur CSS-anchor layout overhead when popup positioning does not use anchors.

Steps to reproduce

1. In a React app with @blocknote/core, @blocknote/react, @blocknote/mantine, and @blocknote/math-block, render this component:

import { BlockNoteSchema, defaultBlockSpecs, defaultInlineContentSpecs } from "@blocknote/core";
import { useCreateBlockNote } from "@blocknote/react";
import { BlockNoteView } from "@blocknote/mantine";
import { createReactInlineMathSpec } from "@blocknote/math-block";
import "@blocknote/mantine/style.css";

const schema = BlockNoteSchema.create({
  blockSpecs: defaultBlockSpecs,
  inlineContentSpecs: {
    ...defaultInlineContentSpecs,
    math: createReactInlineMathSpec(),
  },
});
const initialContent = Array.from({ length: 25 }, (_, row) => ({
  type: "paragraph",
  content: Array.from({ length: 20 }, (_, col) => [
    { type: "text", text: " text ", styles: {} },
    { type: "math", content: `x_{${row * 20 + col}}^2 + y^2 = z^2` },
  ]).flat(),
}));

export default function App() {
  const editor = useCreateBlockNote({ schema, initialContent });
  return <BlockNoteView editor={editor} editable={false}
    formattingToolbar={false} sideMenu={false} slashMenu={false} />;
}

2. Serve a production build and record page load in Chromium's Performance panel.
3. Add .bn-preview-container { anchor-name: none !important; } as a diagnostic override and reload. Layout time drops substantially; all 500 formulas remain rendered.
4. Repeat with editable={true}; the same issue occurs.

BlockNote version

Reproduced on 0.54.0 and current source at 835fabf86ef26c9932075ab47f3d03cf900f276b.

Environment

Chromium 151.0.7922.137, Linux, AMD Ryzen 7 5800X. Production React/Vite fixture, 1280×900 viewport, no CPU throttling.

Additional context

Current-source production builds, three runs per cell; median cumulative CDP LayoutDuration through editor/font readiness:

Median readiness improves from 18.51 to 1.28 seconds in editable mode. The same trigger also reproduces with official block math. No position-anchor, anchor(), or anchor-size() consumer exists in the inspected source; popups currently use absolute positioning.

Contribution

• I'd be interested in contributing a fix for this issue

Start in packages/core/src/editor/Block.css and inspect the .bn-preview-container anchor-name declaration; the issue reports that no anchor consumer exists and that removing it eliminates the Chromium layout cost. Reproduce the provided 500-inline-formula fixture, compare Chromium Performance results with and without the declaration, and confirm all previews remain rendered in both editable and read-only modes.