fix: markdown tables drop empty cells and misalign columns (#1164)

* refactor: extract splitTableRow helper for markdown tables

* fix: keep empty interior cells in markdown tables to preserve columns

* test: splitTableRow keeps empty interior cells

static/js/markdown.js

@@ -5,6 +5,7 @@
  */
 
 import uiModule from './ui.js';
+import { splitTableRow } from './markdown/tableRow.js';
 
 var escapeHtml = uiModule.esc;
 
@@ -535,7 +536,7 @@ export function mdToHtml(src) {
     let html = '<table style="border-collapse: collapse; width: 100%; margin: 10px 0;">';
 
     rows.forEach((row, idx) => {
-      const cells = row.split('|').filter(cell => cell.trim() !== '');
+      const cells = splitTableRow(row);
       if (cells.length === 0) return;
 
       html += idx === 1 ? '<tbody>' : '';

static/js/markdown/tableRow.js

@@ -0,0 +1,18 @@
+// static/js/markdown/tableRow.js
+//
+// Pure helper for splitting a markdown table row into cells. No DOM —
+// safe to import anywhere and to unit-test under node.
+
+// Split a "| a | b | c |" row into trimmed cell strings.
+//
+// Strip only the optional leading/trailing pipe, then split — filtering out
+// every empty cell (the old behaviour) dropped intentionally-empty interior
+// cells too, so "| a |  | c |" collapsed to 2 columns and misaligned with the
+// header.
+export function splitTableRow(row) {
+  return (row || '')
+    .replace(/^\s*\|/, '')
+    .replace(/\|\s*$/, '')
+    .split('|')
+    .map((cell) => cell.trim());
+}

tests/test_markdown_table_row_js.py

@@ -0,0 +1,47 @@
+"""Pin the pure splitTableRow helper (static/js/markdown/tableRow.js).
+
+Driven through `node --input-type=module` (same approach as test_compare_js.py);
+skips when `node` is not installed.
+
+Regression: the old split filtered out every empty cell, so an intentionally
+empty interior cell ("| a |  | c |") collapsed the row to 2 columns and
+misaligned it with the header.
+"""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_HELPER = _REPO / "static" / "js" / "markdown" / "tableRow.js"
+_HAS_NODE = shutil.which("node") is not None
+
+
+def _split(row: str):
+    js = f"""
+    import {{ splitTableRow }} from '{_HELPER.as_posix()}';
+    console.log(JSON.stringify(splitTableRow({json.dumps(row)})));
+    """
+    proc = subprocess.run(
+        ["node", "--input-type=module"],
+        input=js, capture_output=True, text=True, cwd=str(_REPO), timeout=30,
+    )
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout.strip())
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_keeps_empty_interior_cell():
+    assert _split("| a |  | c |") == ["a", "", "c"]
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_rows_without_outer_pipes():
+    assert _split("a | b | c") == ["a", "b", "c"]
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_header_row_unaffected():
+    assert _split("| h1 | h2 | h3 |") == ["h1", "h2", "h3"]