Nested GFM strikethrough breaks into sibling deletes on round-trip

[ChristianMurphy]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Affected package

mdast-util-gfm 3.1.0, mdast-util-to-markdown 2.1.2, mdast-util-from-markdown 2.0.3

Steps to reproduce

Input (9 bytes):

~~a~a~a~~

tree₁ has delete > [text, delete > text, text]:

{
  "type": "delete",
  "children": [
    { "type": "text", "value": "a" },
    { "type": "delete", "children": [{ "type": "text", "value": "a" }] },
    { "type": "text", "value": "a" }
  ]
}

stringify:

~~a~~a~~a~~

tree₂ flattens to three siblings:

{
  "type": "paragraph",
  "children": [
    { "type": "delete", "children": [{ "type": "text", "value": "a" }] },
    { "type": "text", "value": "a" },
    { "type": "delete", "children": [{ "type": "text", "value": "a" }] }
  ]
}

import {fromMarkdown} from 'mdast-util-from-markdown'
import {toMarkdown} from 'mdast-util-to-markdown'
import {gfm} from 'micromark-extension-gfm'
import {gfmFromMarkdown, gfmToMarkdown} from 'mdast-util-gfm'

const opts = {extensions: [gfm()], mdastExtensions: [gfmFromMarkdown()]}
const tree1 = fromMarkdown('~~a~a~a~~', opts)
const text2 = toMarkdown(tree1, {extensions: [gfmToMarkdown()]})
const tree2 = fromMarkdown(text2, opts)

Actual behavior

Adjacent ~~ closing + ~~ opening delimiters parse as sibling strikethrough spans instead of nested ones. Analogous to the nested-emphasis issue but for GFM strikethrough.

Expected behavior

Nested delete with text siblings preserves nesting across round-trip, same as the equivalent emphasis cases should.

Runtime

node v24.13.1

Package manager

No response

Operating system

Ubuntu (WSL2)

Build and bundle tools

No response