fix(vitepress-twoslash): scroll blocking on mobile viewports

[dahlia]

Description

Fix scroll blocking issue on mobile viewports when pages contain Twoslash type hint popups (// ^? queries).

The original issue hypothesized this was a “scroll chaining” problem from overflow-y: auto, but investigation revealed a different root cause: multiple popper elements with position: absolute and transforms were affecting the browser's wheel scroll calculations, causing scroll to stop prematurely (at ~96% of max scroll instead of 100%).

The fix adds content-visibility: auto to popper elements, which tells the browser to optimize rendering of off-screen content and prevents them from interfering with scroll calculations.

Linked issues

Fixes #1234.

Additional context

Testing performed

• Tested on https://optique.dev/why (a page with multiple Twoslash type hints)
• Mobile viewport emulation (390×844, iPhone 12 Pro)
• Before fix: Scroll stopped at 11544px (96%), with 56 backward scroll events detected
• After fix: Scroll reaches 12023px (100%), with 0 backward scroll events

Why overscroll-behavior: contain didn't work

The issue wasn't scroll chaining from inner scrollable elements—it was the popper elements affecting the browser's scroll boundary calculation itself.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.21%. Comparing base (a0b7304) to head (f8f63e0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1235   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files          92       92           
  Lines        7936     7936           
  Branches     1694     1694           
=======================================
  Hits         7556     7556           
  Misses        374      374           
  Partials        6        6           

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[dahlia]

The CI failure on test (22.x, windows-latest) seems unrelated to this PR.

The failing test (injections-side-effects vue) appears to be a snapshot mismatch caused by ANSI escape codes ([7m, [27m) appearing in the output on Windows. This looks like a pre-existing flaky test issue in the core package, rather than something caused by the CSS-only change in this PR.