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+/**
+ * Rehype plugin to restore limited HTML elements inside Markdown table cells.
+ *
+ * ## Problem
+ * The remark/rehype pipeline neutralizes inline HTML as literal text
+ * (remarkLiteralHtml) so that XML/HTML snippets in LLM responses display
+ * as-is instead of being rendered. This causes <br> and <ul> markup in
+ * table cells to show as plain text.
+ *
+ * ## Solution
+ * This plugin traverses the HAST post-conversion, parses whitelisted HTML
+ * patterns from text nodes, and replaces them with actual HAST element nodes
+ * that will be rendered as real HTML.
+ *
+ * ## Supported HTML
+ * - `<br>` / `<br/>` / `<br />` - Line breaks (inline)
+ * - `<ul><li>...</li></ul>` - Unordered lists (block)
+ *
+ * ## Key Implementation Details
+ *
+ * ### 1. Sibling Combination (Critical)
+ * The Markdown pipeline may fragment content across multiple text nodes and `<br>`
+ * elements. For example, `<ul><li>a</li></ul>` might arrive as:
+ *   - Text: `"<ul>"`
+ *   - Element: `<br>`
+ *   - Text: `"<li>a</li></ul>"`
+ *
+ * We must combine consecutive text nodes and `<br>` elements into a single string
+ * before attempting to parse list markup. Without this, list detection fails.
+ *
+ * ### 2. visitParents for Deep Traversal
+ * Table cell content may be wrapped in intermediate elements (e.g., `<p>` tags).
+ * Using `visitParents` instead of direct child iteration ensures we find text
+ * nodes at any depth within the cell.
+ *
+ * ### 3. Reference Comparison for No-Op Detection
+ * When checking if `<br>` expansion changed anything, we compare:
+ *   `expanded.length !== 1 || expanded[0] !== textNode`
+ *
+ * This catches both cases:
+ * - Multiple nodes created (text was split)
+ * - Single NEW node created (original had only `<br>`, now it's an element)
+ *
+ * A simple `length > 1` check would miss the single `<br>` case.
+ *
+ * ### 4. Strict List Validation
+ * `parseList()` rejects malformed markup by checking for garbage text between
+ * `<li>` elements. This prevents creating broken DOM from partial matches like
+ * `<ul>garbage<li>a</li></ul>`.
+ *
+ * ### 5. Newline Substitution for `<br>` in Combined String
+ * When combining siblings, existing `<br>` elements become `\n` in the combined
+ * string. This allows list content to span visual lines while still being parsed
+ * as a single unit.
+ *
+ * @example
+ * // Input Markdown:
+ * // | Feature | Notes |
+ * // |---------|-------|
+ * // | Multi-line | First<br>Second |
+ * // | List | <ul><li>A</li><li>B</li></ul> |
+ * //
+ * // Without this plugin: <br> and <ul> render as literal text
+ * // With this plugin: <br> becomes line break, <ul> becomes actual list
+ */
+
+import type { Plugin } from 'unified';
+import type { Element, ElementContent, Root, Text } from 'hast';
+import { visit } from 'unist-util-visit';
+import { visitParents } from 'unist-util-visit-parents';
+import { BR_PATTERN, LIST_PATTERN, LI_PATTERN } from '$lib/constants/table-html-restorer';
+
+/**
+ * Expands text containing `<br>` tags into an array of text nodes and br elements.
+ */
+function expandBrTags(value: string): ElementContent[] {
+	const matches = [...value.matchAll(BR_PATTERN)];
+	if (!matches.length) return [{ type: 'text', value } as Text];
+
+	const result: ElementContent[] = [];
+	let cursor = 0;
+
+	for (const m of matches) {
+		if (m.index! > cursor) {
+			result.push({ type: 'text', value: value.slice(cursor, m.index) } as Text);
+		}
+		result.push({ type: 'element', tagName: 'br', properties: {}, children: [] } as Element);
+		cursor = m.index! + m[0].length;
+	}
+
+	if (cursor < value.length) {
+		result.push({ type: 'text', value: value.slice(cursor) } as Text);
+	}
+
+	return result;
+}
+
+/**
+ * Parses a `<ul><li>...</li></ul>` string into a HAST element.
+ * Returns null if the markup is malformed or contains unexpected content.
+ */
+function parseList(value: string): Element | null {
+	const match = value.trim().match(LIST_PATTERN);
+	if (!match) return null;
+
+	const body = match[1];
+	const items: ElementContent[] = [];
+	let cursor = 0;
+
+	for (const liMatch of body.matchAll(LI_PATTERN)) {
+		// Reject if there's non-whitespace between list items
+		if (body.slice(cursor, liMatch.index!).trim()) return null;
+
+		items.push({
+			type: 'element',
+			tagName: 'li',
+			properties: {},
+			children: expandBrTags(liMatch[1] ?? '')
+		} as Element);
+
+		cursor = liMatch.index! + liMatch[0].length;
+	}
+
+	// Reject if no items found or trailing garbage exists
+	if (!items.length || body.slice(cursor).trim()) return null;
+
+	return { type: 'element', tagName: 'ul', properties: {}, children: items } as Element;
+}
+
+/**
+ * Processes a single table cell, restoring HTML elements from text content.
+ */
+function processCell(cell: Element) {
+	visitParents(cell, 'text', (textNode: Text, ancestors) => {
+		const parent = ancestors[ancestors.length - 1];
+		if (!parent || parent.type !== 'element') return;
+
+		const parentEl = parent as Element;
+		const siblings = parentEl.children as ElementContent[];
+		const startIndex = siblings.indexOf(textNode as ElementContent);
+		if (startIndex === -1) return;
+
+		// Combine consecutive text nodes and <br> elements into one string
+		let combined = '';
+		let endIndex = startIndex;
+
+		for (let i = startIndex; i < siblings.length; i++) {
+			const sib = siblings[i];
+			if (sib.type === 'text') {
+				combined += (sib as Text).value;
+				endIndex = i;
+			} else if (sib.type === 'element' && (sib as Element).tagName === 'br') {
+				combined += '\n';
+				endIndex = i;
+			} else {
+				break;
+			}
+		}
+
+		// Try parsing as list first (replaces entire combined range)
+		const list = parseList(combined);
+		if (list) {
+			siblings.splice(startIndex, endIndex - startIndex + 1, list);
+			return;
+		}
+
+		// Otherwise, just expand <br> tags in this text node
+		const expanded = expandBrTags(textNode.value);
+		if (expanded.length !== 1 || expanded[0] !== textNode) {
+			siblings.splice(startIndex, 1, ...expanded);
+		}
+	});
+}
+
+export const rehypeRestoreTableHtml: Plugin<[], Root> = () => (tree) => {
+	visit(tree, 'element', (node: Element) => {
+		if (node.tagName === 'td' || node.tagName === 'th') {
+			processCell(node);
+		}
+	});
+};