improvise/harness/tests/contract.spec.js

// The RenderModel→DOM boundary and the pointer's half of the JS→wasm one: the
// two contracts only a browser can establish. Why these and not more, and what
// moved to `node --test web/tests`: context/code/clients.md.

import { test, expect } from "@playwright/test";

// The page's own key, imported rather than spelled: a spec with its own
// copy would pass while reading a key nothing writes (shell.spec.js does
// the same with the handshake window).
import { NOTICE_KEY } from "../../web/shell.js";

async function openApp(page) {
  await page.goto("/app/index.html");
  await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });
  await page.locator("#grid").focus();
}

test.describe("every field the render model projects reaches the screen", () => {
  // The four widgets a user reported as "not working" — the tile bar, the view
  // list, the formula panel and the category sidebar ignoring its flag — were
  // ONE failure at this boundary: a projected field with no element painting
  // it. This is that contract, and it is what catches the fifth.
  test("the widgets the demo document fills are drawn, in the state that fills them", async ({
    page,
  }) => {
    await openApp(page);

    // `rows`, `col_levels`, `row_levels`: the grid itself. web/demo.improv is
    // Product on row, _Measure on column (gotcha 46).
    await expect(page.locator("#grid thead th", { hasText: "Revenue" })).toBeVisible();
    await expect(page.locator("#grid tbody th").first()).toHaveText("Espresso");
    await expect(page.locator("#grid tbody td").first()).not.toHaveText("");

    // `doc_name`, `doc_stored_as`: the title bar. improvise-z754 projected
    // these and nothing painted them for a wave, which `render_surface`'s
    // substring search cannot see — only a loaded page can (improvise-5jna).
    // The dirty marker's own spelling is `web/tests/shell.test.js`: this
    // deployment persists on every accepted batch, so `[+]` is transient.
    await expect(page.locator("#doc-name")).toHaveText(
      "Corner Coffee — 2025 (demo.improv)",
    );

    // `mode`, `hint`, `view_name`, `status`: the header's four facts.
    await expect(page.locator("#mode")).toHaveText("NORMAL");
    await expect(page.locator("#hint")).not.toHaveText("");
    await expect(page.locator("#view-name")).toHaveText("Plan");
    await expect(page.locator("#status")).toContainText("in-page worker");

    // `tiles`: drawn on every frame, as the TUI draws them. The axis label is
    // `Axis::plain_label` here; the three-column one is the TUI's (771e).
    await expect(page.locator("#tiles .tile")).toHaveCount(5);
    await expect(page.locator("#tiles .tile").filter({ hasText: "Product" })).toHaveText(
      "Product Row",
    );

    // `categories` + `category_panel_open`: the flag is the client's answer,
    // and the sidebar was drawn always until it read it (improvise-wsaw).
    await expect(page.locator("#categories")).toBeVisible();
    await expect(page.locator("#categories li").first()).toBeVisible();
    // Open at launch by the shared default, so the first press only FOCUSES:
    // `toggle-panel-and-focus` closes only when open AND focused.
    await page.keyboard.press("Shift+C");
    await expect(page.locator("#mode")).toHaveText("CATEGORIES");
    await expect(page.locator("#categories")).toBeVisible();
    // This is the assertion that catches wsaw — a sidebar drawn unconditionally
    // passes the launch one. Visible-at-launch is the ruling, not the guard.
    await page.keyboard.press("Shift+C");
    await expect(page.locator("#categories")).toBeHidden();
    await page.keyboard.press("Shift+C");
    await expect(page.locator("#categories")).toBeVisible();
    await page.keyboard.press("Escape");

    // `views`: empty unless open, so the page asks no question about the mode.
    await expect(page.locator("#views")).toBeHidden();
    await page.keyboard.press("Shift+V");
    await expect(page.locator("#views li")).toHaveCount(2);
    await expect(page.locator("#views li.active")).toContainText("Plan");
    // The axis line is the shared `view_axis_summary`; its absence would mean
    // a second derivation had crept in.
    await expect(page.locator("#views li").nth(0).locator(".axes")).toHaveText(
      "R:Product P:Quarter",
    );
    await page.keyboard.press("Escape");

    // `formulas`: the third of `Panel::ALL`, on the same terms.
    await page.keyboard.press("Shift+F");
    await expect(page.locator("#formulas li")).toHaveText(["Profit = Revenue - Cost"]);
    await page.keyboard.press("Escape");

    // `help`, `help_page`, `help_titles`: the rows and their keys are
    // `improvise_protocol::help`'s, the same ones the TUI draws. Asserted by
    // TEXT, because an empty `dt` is a visible `dt` and a page drawing the
    // right number of blank rows is the failure this row is here to catch
    // (improvise-igxp). What each page holds is `help.spec.js`.
    await expect(page.locator("#help")).toBeHidden();
    await page.keyboard.press("?");
    await expect(page.locator(".help-tab.active")).toHaveText("Welcome");
    await expect(page.locator(".help-body dt.help-banner")).toHaveText("improvise");
    await page.keyboard.press("l");
    await expect(page.locator(".help-tab.active")).toHaveText("Navigation");
    await expect(page.locator(".help-body dt.help-keys").first()).toHaveText(
      "h j k l ← ↑ → ↓",
    );
    await page.keyboard.press("Escape");
    await expect(page.locator("#help")).toBeHidden();

    // `editing`: what has been typed, drawn into the selected cell with a
    // caret. The decision that an editor is open is made in Rust.
    await page.keyboard.press("i");
    await expect(page.locator("#grid td.typing")).toHaveCount(1);
    // The class alone would pass against a cell drawn blank: what is on the
    // screen is the buffer AND the caret.
    await expect(page.locator("#grid td.typing")).toContainText("▏");
    await page.keyboard.press("Escape");
    await expect(page.locator("#grid td.typing")).toHaveCount(0);
  });

  // The other half of the same contract: a change the client resolved must
  // reach the pixels, not merely the cache. One instance is enough — what
  // each key MEANS is `improvise_protocol::resolve`'s, and tested there.
  test("a resolved change repaints the widget it moved and the grid with it", async ({
    page,
  }) => {
    await openApp(page);
    const product = page.locator("#tiles .tile").filter({ hasText: "Product" });
    const rowHeaders = page.locator("#grid tbody th");
    await expect(rowHeaders).toHaveText(["Espresso", "Latte", "Cold Brew", "Pastry"]);

    await page.keyboard.press("Shift+T");
    for (let i = 0; i < 3; i++) await page.keyboard.press("ArrowRight");
    await expect(product).toHaveClass(/selected/);
    await page.keyboard.press("c");

    await expect(product).toHaveText("Product Col", { timeout: 10_000 });
    // The label alone would be the bar reporting an axis the document has not
    // got: the grid must have moved with it.
    await expect(
      page.locator("#grid thead th").filter({ hasText: "Espresso" }).first(),
    ).toBeVisible();
    await expect(rowHeaders).not.toHaveText(["Espresso", "Latte", "Cold Brew", "Pastry"]);
  });

  // improvise-p7lh. `page_coords` and `totals` were projected to every session
  // and folded correctly, and NO front end read either: the shipped front page
  // showed a grid filtered to Q1 with nothing on screen saying so, and `]`
  // changed every number in silence.
  test("the page bar names the quarter, and both it and the totals follow `]`", async ({
    page,
  }) => {
    await openApp(page);

    // `web/demo.improv` is `Quarter: page, Q1` — the fact this bead is about.
    await expect(page.locator("#pages")).toBeVisible();
    await expect(page.locator("#pages .page-axis")).toHaveText(/Quarter = Q1 \(1\/4\)/);
    // Page is navigable where Filter is not (gotcha 27), and that is what
    // says `[`/`]` will move it.
    await expect(page.locator("#pages .page-axis")).toHaveClass(/navigable/);

    // The totals strip. Q1 Revenue is 18400+24600+9200+11500 — a sum over
    // EVERY row, which is why it is projected and not added up here.
    await expect(page.locator("#grid tfoot th")).toHaveText("Total");
    await expect(page.locator("#grid tfoot td").first()).toHaveText("63,700.00");
    // A total is not a cell anyone may type into.
    await expect(page.locator("#grid tfoot td.editable")).toHaveCount(0);

    await page.keyboard.press("]");

    // BOTH move, and the numbers are Q2's: a bar that showed a stale quarter
    // over fresh numbers is the exact lie this bead names.
    await expect(page.locator("#pages .page-axis")).toHaveText(/Quarter = Q2 \(2\/4\)/, {
      timeout: 10_000,
    });
    await expect(page.locator("#grid tfoot td").first()).toHaveText("66,400.00");
  });
});

test.describe("a pointer event reaches the client and its answer is painted", () => {
  // improvise-7ybz. What a click MEANS is resolved in Rust and tested in
  // `improvise_protocol::resolve`; what this pins is the wiring — the handler
  // is on the element, it passes THAT element's index, and the result repaints.
  test("each clickable surface is wired to its own index", async ({ page }) => {
    await openApp(page);

    // A grid cell: the coordinate is the data row, which group headers do not
    // consume — `gridPlan` counts it and `web/tests/grid_plan.test.js` pins it.
    const cells = page.locator("#grid tbody td");
    await expect(cells.nth(0)).toHaveClass(/selected/);
    await cells.nth(4).click();
    await page.keyboard.press("Escape");
    await expect(cells.nth(4)).toHaveClass(/selected/);
    await expect(page.locator("#grid tbody td.selected")).toHaveCount(1);
    // The cursor a click leaves is the one the keyboard moves from: one
    // cursor, two input paths.
    await page.keyboard.press("ArrowLeft");
    await expect(cells.nth(3)).toHaveClass(/selected/);

    // A tile: the first click SELECTS and does not touch the document; the
    // second, on the same tile, cycles. The user's ruling — clients.md.
    const product = page.locator("#tiles .tile").filter({ hasText: "Product" });
    await product.click();
    await expect(page.locator("#mode")).toHaveText("TILES");
    await expect(product).toHaveClass(/selected/);
    // Waited out, not asserted instantly: a cycle would arrive asynchronously.
    await page.waitForTimeout(500);
    await expect(product).toHaveText("Product Row");
    await product.click();
    await expect(product).toHaveText("Product Col", { timeout: 10_000 });
    await page.keyboard.press("Escape");

    // A category row: focuses the panel, moves its cursor and activates the
    // row — three things from one click, and the index must be the row's own.
    await expect(page.locator("#categories")).toBeVisible();
    const productRow = page.locator("#categories li").filter({ hasText: "Product" });
    await expect(page.locator("#categories li.selected")).toHaveText(/_Index/);
    await productRow.click();
    await expect(page.locator("#mode")).toHaveText("CATEGORIES");
    await expect(page.locator("#categories li.selected")).toHaveText(/Product/);
    await expect(page.locator("#categories li.item")).toHaveText([
      "Espresso",
      "Latte",
      "Cold Brew",
      "Pastry",
    ]);
    // The OTHER arm of the same click, which PR #61 dropped with
    // `pointer.spec.js` and nothing replaced (found by the test-name diff
    // improvise-78v6 asks for): on an ITEM row the rule is "do what Enter
    // does here", which is filter — not expand. Only the status line says
    // which arm ran, so a click wired to the category arm passes everything
    // above and fails here.
    await page.locator("#categories li.item").filter({ hasText: "Latte" }).click();
    await expect(page.locator("#status")).toContainText("Filter: Product = Latte");
    await expect(page.locator("#categories li.selected")).toHaveText("Latte");
    await page.keyboard.press("Escape");

    // A view row (improvise-iioi). The activation must read the CLICKED index,
    // not `view_panel_cursor` — so the cursor must not already be on the row
    // clicked, or both readings give the same answer. `Plan` is index 0 and
    // holds the cursor by default, so move to it and click `Season`.
    await page.keyboard.press("Shift+V");
    await expect(page.locator("#views li.selected")).toContainText("Plan");
    // Switch to Plan by KEYBOARD, leaving the cursor on it. The click then
    // names a different row than the cursor does, which is the whole point.
    await page.keyboard.press("Enter");
    await expect(page.locator("#view-name")).toHaveText("Plan");

    const season = page.locator("#views li").filter({ hasText: "Season" }).first();
    await season.click();
    // Reading the cursor instead of the index re-selects Plan and fails.
    await expect(page.locator("#view-name")).toHaveText("Season");
    await expect(page.locator("#views li.active")).toContainText("Season");
    await expect(page.locator("#views li.selected")).toContainText("Season");

    // A formula row: navigation only, because FormulaPanel binds no Enter and
    // a click mirrors Enter. The Rust guard is on the KEYMAP — clients.md.
    // `#grid input` toHaveCount(0) belongs here and is deliberately absent:
    // it cannot fail, measured by injection (improvise-qf57, clients.md).
    await page.keyboard.press("Shift+F");
    await expect(page.locator("#formulas li").first()).toBeVisible();
    await page.keyboard.press("Escape");
    await page.locator("#formulas li").first().click();
    await expect(page.locator("#mode")).toHaveText("FORMULAS");
    await expect(page.locator("#formulas li.selected")).toHaveCount(1);
  });

  // improvise-0x9u. The neighbouring state the assertion above cannot see: it
  // presses Escape first, so it only ever clicks from a NON-editing page.
  test("a click moves the selection from an editing state too", async ({ page }) => {
    await openApp(page);
    const cells = page.locator("#grid tbody td");

    await cells.nth(0).click();
    await expect(cells.nth(0).locator("input")).toHaveCount(1);

    // No Escape between the two clicks — that is the whole test. The blur used
    // to repaint and `replaceChildren` destroyed the <td> the mousedown landed
    // on, so no click event was ever dispatched and the pointer was dead.
    await cells.nth(4).click();
    await expect(cells.nth(4).locator("input")).toHaveCount(1);
    await expect(cells.nth(0).locator("input")).toHaveCount(0);
    // The third edit of the same repair: `close` restores the DRAWN text, not
    // the seed. `repaint` early-returns while `editing` is set, so a `18400`
    // put back here stays on screen (improvise-0x9u).
    await expect(cells.nth(0)).toHaveText("18,400.00");

    await page.keyboard.press("Escape");
    await expect(cells.nth(4)).toHaveClass(/selected/);
    await expect(page.locator("#grid tbody td.selected")).toHaveCount(1);
  });

  // improvise-b8ot moved these five surfaces onto one handler per stable root.
  // The three tests below are the properties a naive delegation loses, each of
  // which is silent: the page still paints, and only the pointer stops working.
  // The fourth — closing a live editor FIRST — is the test above, which asserts
  // the old editor is gone and the new one is open with no Escape between.
  test("a mousedown alone reaches the grid, because click comes too late", async ({
    page,
  }) => {
    await openApp(page);
    const cells = page.locator("#grid tbody td");

    // MOUSEDOWN ONLY, dispatched with no click behind it. A handler bound to
    // `click` does nothing here, which is the whole of improvise-0x9u: from an
    // editing state the blur repaints and the <td> is gone before click fires.
    await cells.nth(4).dispatchEvent("mousedown");
    await expect(cells.nth(4).locator("input")).toHaveCount(1);

    // And the repaint is SKIPPED on that branch: a repaint would take the
    // <input> away, so its survival is the assertion.
    await page.waitForTimeout(300);
    await expect(page.locator("#grid input")).toHaveCount(1);
  });

  test("the grid prevents its own default, and only over a cell", async ({ page }) => {
    await openApp(page);
    // `window` is last in the bubble path, so it sees what #grid's handler did.
    await page.evaluate(() => {
      window.__defaultPrevented = null;
      window.addEventListener("mousedown", (e) => {
        window.__defaultPrevented = e.defaultPrevented;
      });
    });

    await page.locator("#grid tbody td").nth(1).dispatchEvent("mousedown");
    expect(await page.evaluate(() => window.__defaultPrevented)).toBe(true);

    // A header carried no listener before delegation, so preventing ITS
    // default would be new behaviour — the check must be on the cell, not the
    // root. Without this the first assertion passes for a blanket handler.
    await page.evaluate(() => {
      window.__defaultPrevented = null;
    });
    await page.locator("#grid thead th").last().dispatchEvent("mousedown");
    expect(await page.evaluate(() => window.__defaultPrevented)).toBe(false);
  });

  test("a click's coordinates are read off the node, not closed over", async ({
    page,
  }) => {
    await openApp(page);
    const cells = page.locator("#grid tbody td");
    await expect(cells.nth(0)).toHaveClass(/selected/);

    // Rewriting the attribute must change where the click lands. A per-node
    // closure over `gridPlan`'s answer ignores it and this fails — which is
    // what makes the delegated handler a LOOKUP rather than a second
    // derivation of which data row was clicked (improvise-b8ot).
    await cells.nth(0).evaluate((td) => {
      td.dataset.row = "2";
    });
    await cells.nth(0).dispatchEvent("mousedown");
    await page.keyboard.press("Escape");

    const thirdRow = page.locator("#grid tbody tr").nth(2).locator("td").first();
    await expect(thirdRow).toHaveClass(/selected/);
    await expect(page.locator("#grid tbody td.selected")).toHaveCount(1);
  });
});

test.describe("the selection is on screen", () => {
  // improvise-jq8l: `web/app.js` called `scrollIntoView` NOWHERE, so a selection
  // moved past the bottom of `#grid` simply stayed there, invisible, with no way
  // back but scrolling by hand. Only a browser can see this: it is a fact about
  // `#grid`'s own scroll box, not about the projection, which had already sent
  // the row — the page renders more rows than fit, for the reason below.
  test("a selection driven below the fold scrolls itself into view", async ({ page }) => {
    // `harness/fixtures/tall.improv`: 30 rows in three groups, which is more
    // than the box can show because `ROW_PX` (26) under-estimates the real
    // 28.6px row and inline group rows are deliberately not charged for.
    await page.goto("/app/index.html?doc=tall-scroll.improv&seed=tall.improv");
    await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });
    await expect(page.locator("#view-name")).toHaveText("Tall");
    await expect(page.locator("#grid tbody th[scope=row]").first()).toHaveText("R01");
    await page.locator("#grid").focus();

    // Not vacuous by construction: if `#grid` does not overflow there is
    // nothing to scroll and every assertion below passes on a broken page.
    const overflow = await page
      .locator("#grid")
      .evaluate((g) => g.scrollHeight - g.clientHeight);
    expect(overflow).toBeGreaterThan(0);

    for (let i = 0; i < 25; i++) await page.keyboard.press("ArrowDown");

    // The selected cell must be inside the box the user can see. Measured
    // against `#grid`'s own rect rather than the window's, because `#grid` is
    // the scroll container — the page itself does not scroll.
    await expect
      .poll(async () =>
        page.locator("#grid").evaluate((g) => {
          const sel = g.querySelector("td.selected");
          if (!sel) return "no selected cell";
          const cell = sel.getBoundingClientRect();
          const box = g.getBoundingClientRect();
          return cell.bottom <= box.bottom + 1 && cell.top >= box.top - 1;
        }),
      )
      .toBe(true);
    // And it really did have to move: a page that never scrolls leaves this 0.
    expect(await page.locator("#grid").evaluate((g) => g.scrollTop)).toBeGreaterThan(0);
  });
});

// The page's own chrome: what a visitor lands on, and where the prompt is.
// Only a browser can answer either — both are facts about layout and about
// storage that survives a reload. What these CANNOT see is the rest of the
// styling; that residue is named in context/code/clients.md.
test.describe("the shell puts the application first", () => {
  const notice = (page) => page.locator("#backend-notice");
  const stored = (page) => page.evaluate((k) => localStorage.getItem(k), NOTICE_KEY);

  // The deployment notice is ten lines of configuration reference — ?doc=,
  // ?seed=, ?ws=, wss:// and reverse proxies — and it was above the fold on
  // every visit. See context/test-evidence/clients.md.
  test("the deployment notice starts collapsed and remembers being opened", async ({
    page,
  }) => {
    await page.goto("/app/index.html?doc=notice-memory.improv");
    await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });

    // The one sentence a visitor needs is never collapsed; the reference is.
    await expect(page.locator("#backend-notice .notice-lede")).toBeVisible();
    await expect(page.locator("#backend-notice .notice-body")).toBeHidden();

    // BOTH directions, because a page that ignores storage passes the first
    // half and a page that always opens passes the second (design-principles
    // §7: assert the direction the default cannot supply).
    await notice(page).locator("summary").click();
    await expect(page.locator("#backend-notice .notice-body")).toBeVisible();
    // `toggle` is dispatched as a task, not synchronously with the click, so
    // the answer must be OBSERVED as stored before a reload can be said to
    // have read it back. Without this the reload races the write.
    await expect.poll(() => stored(page)).toBe("open");
    await page.reload();
    await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });
    await expect(page.locator("#backend-notice .notice-body")).toBeVisible();

    await notice(page).locator("summary").click();
    await expect(page.locator("#backend-notice .notice-body")).toBeHidden();
    await expect.poll(() => stored(page)).toBe("closed");
    await page.reload();
    await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });
    await expect(page.locator("#backend-notice .notice-body")).toBeHidden();
  });

  // The `<details>` is the browser's control, and the page's own keydown
  // handler claims Enter. Without the guard a keyboard visitor could focus
  // the summary and not open it. See context/test-evidence/clients.md.
  test("the notice opens from the keyboard, which the grid's own handler must not claim", async ({
    page,
  }) => {
    await page.goto("/app/index.html?doc=notice-keyboard.improv");
    await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });
    await expect(page.locator("#backend-notice .notice-body")).toBeHidden();

    await notice(page).locator("summary").focus();
    await page.keyboard.press("Enter");
    await expect(page.locator("#backend-notice .notice-body")).toBeVisible();
  });

  // The prompt was a small line in the page's bottom-left corner, 800px from
  // the cell being typed into. See context/test-evidence/clients.md.
  test("the minibuffer is raised over the grid rather than filed under it", async ({
    page,
  }) => {
    await page.goto("/app/index.html?doc=minibuffer-place.improv");
    await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });
    await page.locator("#grid").focus();
    await expect(page.locator("#minibuffer")).toBeHidden();

    // `a` is the one text-entry room this client can both enter and commit in
    // (`keys.spec.js` drives the formula one). The precondition is that it
    // really opened, or every measurement below is of a hidden element.
    await page.keyboard.press("a");
    await expect(page.locator("#mode")).toHaveText("INSERT");
    await expect(page.locator("#minibuffer")).toBeVisible();

    const boxes = await page.evaluate(() => {
      const rect = (id) => {
        const { top, bottom, left, right } = document
          .getElementById(id)
          .getBoundingClientRect();
        return { top, bottom, left, right };
      };
      return {
        mb: rect("minibuffer"),
        grid: rect("grid"),
        viewport: { height: window.innerHeight, width: window.innerWidth },
        mbFont: parseFloat(getComputedStyle(document.getElementById("minibuffer")).fontSize),
        bodyFont: parseFloat(getComputedStyle(document.body).fontSize),
      };
    });

    // On screen without scrolling: the property a bottom-of-document line
    // does not have once the page is taller than the window.
    expect(boxes.mb.bottom).toBeLessThanOrEqual(boxes.viewport.height);
    expect(boxes.mb.top).toBeGreaterThanOrEqual(0);
    // And OVER the grid, not in a strip beneath it. This is what separates
    // "raised" from "moved": a static bottom bar cannot overlap #grid at all.
    expect(boxes.mb.top).toBeLessThan(boxes.grid.bottom);
    expect(boxes.mb.bottom).toBeGreaterThan(boxes.grid.top);
    // The one prominence claim a browser can check. It was body-sized before.
    expect(boxes.mbFont).toBeGreaterThan(boxes.bodyFont);

    await page.keyboard.press("Escape");
    await expect(page.locator("#minibuffer")).toBeHidden();
  });
});

// The stylesheet's own cascade, which no other test here can reach: an
// error is a colour now, where it used to be a colour AND an opacity, so
// the order of two equally-specific rules decides it. See
// context/test-evidence/clients.md.
test.describe("the grid's semantic colours survive the cascade", () => {
  test("an error cell stays an error cell when it is also dimmed or selected", async ({
    page,
  }) => {
    await page.goto("/app/index.html?doc=error-colour.improv");
    await expect(page.locator("#grid table")).toBeVisible({ timeout: 20_000 });

    // The question is about the STYLESHEET, so the combinations are put to it
    // directly: the document that produces each of them is a separate matter,
    // and a fixture per combination would test the fixture.
    const colours = await page.evaluate(() => {
      const row = document.querySelector("#grid tbody tr");
      const read = (classes) => {
        const td = document.createElement("td");
        td.className = classes;
        td.textContent = "#err";
        row.appendChild(td);
        const colour = getComputedStyle(td).color;
        td.remove();
        return colour;
      };
      return {
        // An aggregate error, which is the common case: `editable` is false
        // for every cell that is not a stored record.
        dimmed: read("error"),
        selected: read("error selected"),
        typing: read("error typing"),
        // What "dimmed" must NOT be, read off a plain unsettable cell.
        dim: read(""),
        // …and what it must be, read off an error the dimming cannot reach.
        error: read("error editable"),
      };
    });

    expect(colours.error).not.toBe(colours.dim);
    expect(colours.dimmed).toBe(colours.error);
    expect(colours.selected).toBe(colours.error);
    expect(colours.typing).toBe(colours.error);
  });
});