dirty monkey-patch of CodeMirror's EditorView.bidiSpans() to force math blocks into LTR.

overrideBidiSpans.ts

import { BidiSpan, EditorView } from "@codemirror/view";
import { Line } from "@codemirror/state";

// Monkey patch the bidiSpans method to override the bidi spans for math blocks

/** Create a new BidiSpan */
const bidiSpan = (from: number, to: number, dir: number) =>
  // @ts-expect-error - BidiSpan constructor is private
  new BidiSpan(from, to, dir);


const mathRegex = /\$\$?.*?\$\$?/g;

/** Extract the math blocks as BidiSpans */
function getMathRanges(text: string): BidiSpan[] {
  const ranges: [number, number][] = [];
  let match = mathRegex.exec(text);
  while (match) {
    ranges.push([match.index, match.index + match[0].length]);
    match = mathRegex.exec(text)
  }
  return ranges.map(([from, to]) => bidiSpan(from, to, 2));
}

const bidiSpans = EditorView.prototype.bidiSpans;

// This is inefficient - it's possible to override all spans in one pass.
function overrideSpans(spans: readonly BidiSpan[], newSpan: BidiSpan) {
  const newSpans: BidiSpan[] = [];

  let i = 0;
  while (i < spans.length && spans[i].to < newSpan.from) {
    newSpans.push(spans[i]);
    i++;
  }
  if (i === spans.length) {
    return newSpans;
  }
  const span = spans[i];
  const preSpan = bidiSpan(span.from, newSpan.from, span.dir);

  newSpans.push(preSpan);
  newSpans.push(newSpan);
  i++;

  while (i < spans.length && spans[i].to <= newSpan.to) {
    i++;
  }
  if (i === spans.length) {
    return newSpans;
  }

  const postSpan = bidiSpan(newSpan.to, spans[i].to, span.dir);
  newSpans.push(postSpan);
  i++;

  while (i < spans.length) {
    newSpans.push(spans[i]);
    i++;
  }

  // Make sure last span is RTL
  if (newSpans.length > 0 && newSpans[newSpans.length - 1].level === 2) {
    newSpans.push(
      bidiSpan(newSpans[newSpans.length - 1].to, newSpan.to, 1)
    )
  }

  return newSpans;
}

export const overrideBidiSpans = () => {
  EditorView.prototype.bidiSpans = function (line: Line): readonly BidiSpan[] {
    let spans = bidiSpans.call(this, line);
    const mathRanges = getMathRanges(line.text);

    // Override bidi spans such that math blocks are treated as LTR
    for (const mathRange of mathRanges) {
      spans = overrideSpans(spans, mathRange);
    }

    return spans;
  }
}