Agents window Markdown editor: Most of a thematic break maps clicks to the next paragraph

Testing #326540

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: Insiders with the experimental Markdown editor
- OS Version: macOS
- Audited package revision: microsoft/vscode-packages@1d81418d61ad769f154579196a51a9364c105e2d

### Steps to Reproduce

1. Open this Markdown document in **Editing** mode:

```markdown
Before

---

After
```

2. While the thematic break is rendered as a horizontal rule, click near its center or right side.

### Expected

Clicking anywhere on the rendered rule activates that block, reveals `---`, and positions the caret at the nearest editable location in the rule.

### Actual

Only clicking near the far-left edge maps into the thematic-break source. Clicking the center or right side moves the caret to the beginning of `After`.

### Impact

Most of the visible rule is a misleading hit target. Users trying to edit or delete the rule are moved into unrelated content and may start typing in the following paragraph.