fix: respect tab stops in list continuation indentation

[GreedyC]

Marked version: 18.0.14 (current master)

Markdown flavor: CommonMark and GitHub Flavored Markdown

Description

Related to the list/tab indentation discussion in #4050. This addresses the continuation-line tab-stop calculation only, not the separate preservation of leading tabs inside fenced code.

Expectation

In a list continuation line, a tab advances to the next four-column tab stop. For example, two spaces followed by a tab occupy four columns, not six:

marked.parse('- foo\n\n  \tbar');

After removing the two-column list indentation, bar remains a paragraph rather than an indented code block.

Result before this change

The leading whitespace replacement unconditionally added four spaces per tab. The example above therefore became an indented code block:

<ul>
<li><p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ul>

Change

Reuse expandTabs for the leading whitespace of non-pedantic continuation lines, starting at column zero. Interior tabs and the pedantic branch remain untouched. The existing first-line path already uses this helper with the bullet-column offset.

Four new assertions failed on the unmodified implementation before the fix. The completed regression suite covers both parser flavors, unordered and one-/two-digit ordered markers, genuine indented code, interior tabs, tab-only blank lines, and nested lists. The reference CommonMark parser also confirms the paragraph-versus-code distinction for the reproducer.

Verification

• npm test passed on macOS with Node 20.20.2, Node 22.23.3, and Node 24.16.0: 1,863 spec tests and 207 unit tests per run, plus UMD/CJS, type/export checks, lint, and documentation build.
• Local GitHub Copilot CLI read-only review found no actionable defects. This is not an upstream reviewer approval.
• Fork preflight passed all six full-suite jobs on Linux, Windows, and macOS with Node 20/24, checking out this PR's exact commit 05af44198272295a8e6255de053aeffc530684a1. npm scripts use Bash so Windows Node 20 expands the existing test/unit/*.test.js glob. A separate unchanged-baseline job confirms that the default Windows shell fails to expand that glob; no upstream package scripts were changed for this unrelated limitation.
• Upstream Tests awaits maintainer workflow approval. Vercel reports "Authorization required to deploy" and its bot requests MarkedJS team authorization; it has not reported a build failure. Snyk passed. Fork results are not upstream approval.

AI assistance: Codex was used for implementation and verification, and GitHub Copilot CLI for an additional local review.

Contributor

• Tests exist to ensure functionality and minimize regression.
• If submitting a new feature, it has been documented in the appropriate places. (Not applicable: bug fix.)

Committer

• CI is green (no forced merge required).
• Squash and Merge PR following conventional commit guidelines.

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[UziTech]

These tests should be moved to test/specs/new