feat: complete shared platform UI contracts

webui/tests/data-grid-actions.test.tsx

@@ -129,3 +129,21 @@ const externalShortcutMarkup = renderToStaticMarkup(
 );
 assertEqual(externalShortcutMarkup.includes("Target alpha"), true, "an external count shortcut synchronizes the grid query");
 assertEqual(externalShortcutMarkup.includes("Ordinary row"), false, "the synchronized query does not disagree with visible rows");
+
+const fixedCoverMarkup = renderToStaticMarkup(
+  <DataGrid
+    id="fixed-cover-regression"
+    rows={[{ id: "row", first: "First", second: "Second" }]}
+    columns={[
+      { id: "first", header: "First", width: 100, value: (row) => row.first },
+      { id: "second", header: "Second", width: 200, value: (row) => row.second },
+      { id: "actions", header: "Actions", width: 72, sticky: "end", value: () => "" }
+    ]}
+    getRowKey={(row) => row.id}
+  />
+);
+assertEqual(
+  fixedCoverMarkup.includes("data-grid-buffer-cell"),
+  false,
+  "cover grids never render a synthetic blank column"
+);