Type: Bug

Summary

vscode/extensions/markdown-language-features/preview-src/scroll-sync.ts uses a binary search in getLineElementsAtPageOffset to map preview scroll positions back to editor line numbers. This algorithm assumes that all code-line elements have monotonically increasing bounding boxes.

When some lines are wrapped inside an element with display:none, their bounding boxes become (top = 0, height = 0). This breaks ordering assumptions and causes the binary search to return incorrect elements, resulting in scroll‑sync mismatches.

Steps to Reproduce

<div class="lang-ko">

- First page Korean AAA
- First page Korean BBB 
- First page Korean CCC
- First page Korean DDD

</div>

<div class="lang-en" style="display: none">

- First page English AAA
- First page English BBB
- First page English CCC
- First page English DDD

</div>

---

<div class="lang-ko">

- Second page Korean AAA
- Second page Korean BBB
- Second page Korean CCC
- Second page Korean DDD

</div>
<div class="lang-en"  style="display: none">

- Second page English AAA
- Second page English BBB
- Second page English CCC
- Second page English DDD

</div>

---

<div class="lang-ko">

- Third page Korean AAA
- Third page Korean BBB
- Third page Korean CCC
- Third page Korean DDD

</div>
<div class="lang-en"  style="display: none">

- Third page English AAA
- Third page English BBB
- Third page English CCC
- Third page English DDD

</div>

1. Open Markdown preview.
2. Move the cursor in the editor or double‑click inside the preview.
3. Initially, scroll sync may appear to work, but after a few cursor moves or repeated clicking in the preview, the mapping gradually becomes increasingly incorrect and eventually jumps to wrong lines after the hidden section.

Expected Behavior

• Hidden elements should not participate in scroll‑sync calculations.
• Scroll sync should remain correct even if parts of the DOM are visually removed.

Proposed Fix

Related discussions:

• markdown preview: trouble with <details> blocks  #175353
• Scroll sync not working properly for collapsed markdown details blocks #176538
• Fix for <details> visibility: 71504b5
  • However, only <details> was patched; other hidden elements still break scroll‑sync.

Add a visual‑presence filter before binary search:

function isVisuallyPresent(line: CodeLineElement): boolean {
  const style = window.getComputedStyle(line.element);

  if (style.display === 'none' || style.visibility === 'hidden') {
    return false;
  }

  const bounds = getElementBounds(line);
  return bounds.height > 0 || bounds.width > 0;
}

const lines = getCodeLineElements(documentVersion)
  .filter(x => x.isVisible && isVisuallyPresent(x));

If calling getComputedStyle is considered too expensive, the check could rely solely on bounding‑box validation (maybe?).

I am not familiar with contributing to VSCode, and the fix described above is only a conceptual suggestion rather than a fully working implementation. I am sharing this analysis in case it may help guide a potential improvement 😄

VS Code version: Code 1.106.3 (bf9252a, 2025-11-25T22:28:18.024Z)
OS version: Windows_NT x64 10.0.26100
Modes: