Use a passed HTMLTableCellElement as the cell directly

When a cell value is a <td>/<th>, append it to the row as-is instead of
wrapping it in a generated <td>, which would have produced invalid nested
cells. Any other HTMLElement still gets wrapped as before.

abstract-table.ts

@@ -143,7 +143,8 @@ export abstract class Table<TRow extends object> {
      * Creates a row, populated with data, for displaying in the table.
      * @param data The data in the shape of `TRow`, with values populated.
      * @return A HTMLTableRowElement that can then be set on a table.
-     * @remarks If a cell value is an HTMLElement it is appended to the cell. A DOM node can only
+     * @remarks If a cell value is an HTMLTableCellElement (a `<td>`/`<th>`) it is used as the cell
+     * directly; any other HTMLElement is appended inside a generated `<td>`. A DOM node can only
      * exist in one place, so passing the same element instance to multiple rows moves it to the
      * last row only. Pass a fresh element (or `el.cloneNode(true)`) per row. But keep in mind that
      * el.cloneNode(true) does not keep event listeners, so if event listeners are used it is best
@@ -153,14 +154,20 @@ export abstract class Table<TRow extends object> {
         const row = document.createElement("tr");
 
         for (const column of this.columns) {
-            const cell: HTMLTableCellElement = document.createElement("td");
             const value = data[column.key]
 
+            if (value instanceof HTMLTableCellElement) {
+                row.append(value);        // already a cell, use it as-is
+                continue;
+            }
+
+            const cell: HTMLTableCellElement = document.createElement("td");
+
             // Element values are inserted as children, not stringified.
             if (value instanceof HTMLElement) {
                 cell.appendChild(value);
                 row.append(cell);
-                continue;                 // skip the textContent path that would wipe it
+                continue; // skip the textContent path that would wipe it
             }
 
             let cellValue = "";