Add tests verifying that #18817 has been fixed (#35744)

e2e/test/scenarios/visualizations-tabular/reproductions/18976-pivot-table-columns.cy.spec.js

-import { restore, visitQuestionAdhoc } from "e2e/support/helpers";
+import { restore, rightSidebar, visitQuestionAdhoc } from "e2e/support/helpers";
 import { SAMPLE_DB_ID } from "e2e/support/cypress_data";
+import { SAMPLE_DATABASE } from "e2e/support/cypress_sample_database";
+
+const { PEOPLE_ID, PEOPLE } = SAMPLE_DATABASE;
 
 const questionDetails = {
   display: "table",
@@ -18,7 +21,7 @@ const questionDetails = {
   },
 };
 
-describe("issue 18976", () => {
+describe("issue 18976, 18817", () => {
   beforeEach(() => {
     restore();
     cy.signInAsAdmin();
@@ -30,4 +33,34 @@ describe("issue 18976", () => {
     // eslint-disable-next-line no-unscoped-text-selectors -- deprecated usage
     cy.findByText("Showing 1 row");
   });
+
+  it("should not keep orphan columns rendered after switching from pivot to regular table (metabase#18817)", () => {
+    cy.createQuestion(
+      {
+        query: {
+          "source-table": PEOPLE_ID,
+          aggregation: [["count"]],
+          breakout: [
+            ["field", PEOPLE.NAME],
+            ["field", PEOPLE.SOURCE],
+          ],
+          limit: 5,
+        },
+        database: SAMPLE_DB_ID,
+        display: "table",
+      },
+      { visitQuestion: true },
+    );
+
+    cy.findByTestId("qb-header").button("Summarize").click();
+    rightSidebar()
+      .findByLabelText("Source")
+      .findByRole("button", { name: "Remove dimension" })
+      .click();
+
+    cy.findAllByTestId("header-cell")
+      .should("have.length", 2)
+      .and("contain", "Name")
+      .and("contain", "Count");
+  });
 });