Bug 1688532: Don't use the cached column header on a previous table cell if that cell starts in a different column. r=eeejay

Differential Revision: https://phabricator.services.mozilla.com/D103146

accessible/generic/TableCellAccessible.cpp

@@ -67,7 +67,20 @@ Accessible* TableCellAccessible::PrevColHeader() {
 
     // Check whether the previous table cell has a cached value.
     cachedHeader = cache.GetWeak(tableCell, &inCache);
-    if (inCache && cell->Role() != roles::COLUMNHEADER) {
+    if (
+        // We check the cache first because even though we might not use it,
+        // it's faster than the other conditions.
+        inCache &&
+        // Only use the cached value if:
+        // 1. cell is a table cell which is not a column header. In that case,
+        // cell is the previous header and cachedHeader is the one before that.
+        // We will return cell later.
+        cell->Role() != roles::COLUMNHEADER &&
+        // 2. cell starts in this column. If it starts in a previous column and
+        // extends into this one, its header will be for the starting column,
+        // which is wrong for this cell.
+        // ColExtent is faster than ColIdx, so check that first.
+        (tableCell->ColExtent() == 1 || tableCell->ColIdx() == colIdx)) {
       if (!cachedHeader || !cachedHeader->IsDefunct()) {
         // Cache it for this cell.
         cache.Put(this, RefPtr<Accessible>(cachedHeader));

accessible/tests/mochitest/table/test_headers_table.html

@@ -426,6 +426,37 @@
 
       testHeaderCells(headerInfoMap);
 
+      // ////////////////////////////////////////////////////////////////////////
+      // Ensure correct column headers after colspan in a previous row.
+      headerInfoMap = [
+        {
+          cell: "t11r1c1",
+          columnHeaderCells: [],
+          rowHeaderCells: [],
+        },
+        {
+          cell: "t11r1c2",
+          columnHeaderCells: [],
+          rowHeaderCells: [],
+        },
+        {
+          cell: "t11r2c1_2",
+          columnHeaderCells: ["t11r1c1"],
+          rowHeaderCells: [],
+        },
+        {
+          cell: "t11r3c1",
+          columnHeaderCells: ["t11r1c1"],
+          rowHeaderCells: [],
+        },
+        {
+          cell: "t11r3c2",
+          columnHeaderCells: ["t11r1c2"],
+          rowHeaderCells: [],
+        },
+      ];
+      testHeaderCells(headerInfoMap);
+
       SimpleTest.finish();
     }
 
@@ -707,5 +738,19 @@
       <td headers="t10_males t10_todd t10_10km" id="t10_r3c4">50:35</td>
     </tr>
   </table>
+
+  <table id="table11">
+    <tr>
+      <th id="t11r1c1">a</th>
+      <th id="t11r1c2">b</th>
+    </tr>
+    <tr>
+      <td id="t11r2c1_2" colspan="2"></td>
+    </tr>
+    <tr>
+      <td id="t11r3c1">e</td>
+      <td id="t11r3c2">f</td>
+    </tr>
+  </table>
 </body>
 </html>