Folding Fails When Divs Contain Headings

Bug description

(reposted from https://github.com/posit-dev/positron/issues/11625)

Folding breaks when a Quarto document contains headings inside div blocks.

Steps to reproduce

1. Open a Quarto (.qmd) file in Positron
2. Add the code sample below
3. Attempt to fold ### Heading B

Test Code:

## Test

Testing, testing, ... 🎤

### Heading A

Fabio vel iudice vincam, sunt in culpa qui officia. Inmensae subtilitatis, obscuris et malesuada fames. Ambitioni dedisse scripsisse iudicaretur. Nec dubitamus multa iter quae et nos invenerat. Petierunt uti sibi concilium totius Galliae in diem certam indicere.

### Heading B

Petierunt uti sibi concilium totius Galliae in diem certam indicere. Morbi fringilla convallis sapien, id pulvinar odio volutpat. A communi observantia non est recedendum.

::: {#thm-line}
## Line

The equation of any straight line can be written as:

$$
y = mx + b
$$
:::

#### Another Heading

Cum sociis natoque penatibus et magnis dis parturient. Curabitur blandit tempus ardua ridiculus sed magna. Salutantibus vitae elit libero, a pharetra augue.

Actual behavior

Folding does not work correctly when headings appear inside div blocks.

Expected behavior

Folding should work correctly regardless of whether headings appear inside div blocks.

Your environment

Positron Version: 2026.02.0 build 139
Code - OSS Version: 1.106.0
Commit: 4b472e9bf40ca88f886fe9c7fcfbb9ee702fccd0
Date: 2026-01-29T04:47:19.126Z
Electron: 37.7.0
Chromium: 138.0.7204.251
Node.js: 22.20.0
V8: 13.8.258.32-electron.0
OS: Linux x64 6.18.7-arch1-1

Start by reproducing the supplied .qmd example in Positron and observe folding for Heading B and the heading inside the div block. Trace the folding behavior from the Quarto document handling used by the editor. Done means headings fold correctly whether or not they appear inside div blocks.