fix: the same row again is the same selection

e2e/specs/grid-selection.spec.js

@@ -57,6 +57,33 @@ async function cellAt(toplevel, position, cell) {
 }
 
 /** The number cell selects and never opens an editor, which is why every gesture below lands on it. */
+/** How many times the grid has been committed, which `CueList.tsx` counts on the element. */
+function gridRenders() {
+  return browser.execute(() => Number(document.documentElement.dataset.gridRenders) || 0);
+}
+
+/** Wait until the grid stops drawing on its own, so the count below is about the click. */
+async function settle() {
+  let last = -1;
+  for (;;) {
+    const now = await gridRenders();
+    if (now === last) {
+      return;
+    }
+    last = now;
+    await browser.pause(400);
+  }
+}
+
+/** The list position the cursor is on, or null. */
+function cursorRow() {
+  return browser.execute(() => {
+    const row = document.querySelector(".cuelist__row--active");
+    const text = row?.querySelector(".cuelist__pos")?.textContent;
+    return text === undefined || text === null ? null : Number(text);
+  });
+}
+
 async function clickRow(toplevel, position, modifier = null) {
   const at = await cellAt(toplevel, position, ".cuelist__pos");
   if (modifier === null) {
@@ -226,4 +253,28 @@ describe("the grid's selection gestures", () => {
     });
     expect(await present(".statusbar__dirty")).toBe(false);
   });
+
+  it("draws nothing again when the click lands on the row the cursor already holds", async () => {
+    // Interface-spec 7.2: announcing the same selection again is a redundant render in React, and a
+    // render that happened for nothing cannot be seen from outside. The grid counts its own commits
+    // on the element, which is what makes this readable at all. See BACKLOG.md N159.
+    await clickRow(toplevel, 2);
+    await waitFor(async () => ((await cursorRow()) === 2 ? 1 : null), {
+      timeout: 15000,
+      message: "the cursor to land on the second row",
+    });
+    await settle();
+
+    const before = await gridRenders();
+    await clickRow(toplevel, 2);
+    await browser.pause(1000);
+    expect(await gridRenders()).toBe(before);
+
+    // The control, and it is what says the counter counts: another row does draw again.
+    await clickRow(toplevel, 3);
+    await waitFor(async () => ((await gridRenders()) > before ? 1 : null), {
+      timeout: 15000,
+      message: "a click on another row to draw the grid again",
+    });
+  });
 });

e2e/wdio.conf.js

@@ -44,7 +44,7 @@ import {
  * Every spec that exists must run. WebdriverIO does not reliably fail a run that executed nothing,
  * so the count is asserted here. Bump it when you add a test; see e2e/README.md.
  */
-const EXPECTED_TESTS = 515;
+const EXPECTED_TESTS = 516;
 
 /** How long mocha lets one test live. Every wait inside a test must be shorter. See N165. */
 const TEST_LIMIT_MS = 60000;

src/components/CueList.tsx

@@ -89,6 +89,13 @@ export default function CueList({
 }: CueListProps) {
   const listRef = useRef<HTMLDivElement>(null);
   const editorRef = useRef<HTMLTextAreaElement>(null);
+  // Every commit of the grid, counted where a check can read it. Interface-spec 7.2 asks a click on
+  // the row the cursor already holds to change nothing, and a render that happened for nothing is
+  // invisible from outside by definition. No dependency list: this counts commits, not values.
+  useEffect(() => {
+    const root = document.documentElement;
+    root.dataset.gridRenders = String((Number(root.dataset.gridRenders) || 0) + 1);
+  });
   const [scrollTop, setScrollTop] = useState(0);
   const [viewport, setViewport] = useState(0);
   const [editing, setEditing] = useState<number | null>(null);

src/hooks/useCueSelection.ts

@@ -65,7 +65,11 @@ export function useCueSelection(count: number, openId: number): CueSelection {
       setSelected(runBetween(anchor.current, index));
     } else if (how === "plain") {
       anchor.current = index;
-      setSelected(new Set([index]));
+      // The same one row again is the same selection. A new Set with the same member is a new
+      // object, and React renders everything that reads it for nothing (interface-spec 7.2).
+      setSelected((current) =>
+        current.size === 1 && current.has(index) ? current : new Set([index]),
+      );
     }
   }, []);