fix: keep the flash line to its one line at any message length (closes #252)

The flash line reserves one line, so a message that wrapped pushed the
screen below it down. #flash-msg no longer wraps: text too long for it is
cut with an ellipsis, and showFlash() puts the whole message in its title.
Every message is also reworded to at most 50 characters so none is cut,
and the add-token screens flash a fixed line for any error other than the
two lookup messages, logging the detail.

A new end-to-end test writes a message several lines long into the line
and fails if the line or the screen below it moves.

Model: opus-5-5

TODO.md

@@ -45,6 +45,25 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: The flash line keeps to the one line it reserves at any message
+  length ([#252](https://git.eeqj.de/sneak/AutistMask/issues/252)). A message
+  that wrapped pushed the whole screen below it down. `#flash-msg` no longer
+  wraps: text too long for the line is cut with an ellipsis, and `showFlash()`
+  puts the whole message in the line's title. Every message is also reworded to
+  at most 50 characters so none is cut; none carries a wallet name or text from
+  a server, and the add-token screens flash a fixed line for any error other
+  than a contract that is not a token. A new test in `tests/e2e/run.js` puts a
+  message several lines long on the line and fails if the line or the screen
+  below it moves. The two approval-screen error boxes are left to
+  [#297](https://git.eeqj.de/sneak/AutistMask/issues/297).
+
+- 2026-10-04: A method the wallet does not implement is refused with EIP-1193
+  code `4200` ([#279](https://git.eeqj.de/sneak/AutistMask/issues/279)). The
+  background's `Unsupported method: <method>` error carried no code, so a site
+  probing for an optional method could not tell "not implemented" from "the call
+  failed". The message is unchanged; the background's other errors with no code
+  are untouched.
+
 - 2026-10-04: Settings lists the sites connected without "Remember", and
   removing a site there disconnects it
   ([#406](https://git.eeqj.de/sneak/AutistMask/issues/406)). Such a connection

src/background/index.js

@@ -932,7 +932,9 @@ async function handleRpc(method, params, origin) {
         }
     }
 
-    return { error: { message: "Unsupported method: " + method } };
+    // EIP-1193 4200 lets a site tell "this wallet does not implement that"
+    // from "that call failed", and fall back.
+    return { error: { code: 4200, message: "Unsupported method: " + method } };
 }
 
 // The body of eth_sendTransaction, from the connection check through to the

src/content/inpage.js

@@ -31,11 +31,12 @@
     // an error instead of accepting the refusal.
     //
     // Whatever code arrived is passed through verbatim rather than being
-    // matched against a list: the extension emits 4001, 4100 and 4902 today,
+    // matched against a list: the extension emits codes such as 4001, 4100,
-    // and a code this file has never heard of is still the truth about what
+    // 4200 and 4902, and a code this file has never heard of is still the
-    // happened. An error reported with no code at all stays a plain Error —
+    // truth about what happened. An error reported with no code at all stays
-    // a ProviderRpcError whose `code` is undefined would advertise a
+    // a plain Error — a ProviderRpcError whose `code` is undefined would
-    // conformance it does not have. `message` is untouched in every case.
+    // advertise a conformance it does not have. `message` is untouched in
+    // every case.
     function toPageError(error) {
         const message = (error && error.message) || "Request failed";
         if (error && error.code !== undefined && error.code !== null) {

src/popup/dustThreshold.js

@@ -19,13 +19,9 @@
 // that the user did not type — the same silent substitution the visible
 // rejection message exists to end.
 
-// Must render on ONE line of #flash-msg, whose reserved height
+// Must render on ONE line of #flash-msg; see showFlash() in
-// (min-h-[1.25rem]) is exactly one line at text-xs. A string long enough to
+// src/popup/views/helpers.js for how long that is.
-// wrap to two lines pushes the settings view down, which the No Layout Shift
+const DUST_THRESHOLD_MESSAGE = "Enter a whole number of gwei, zero or greater.";
-// policy forbids. Do not lengthen this without re-running the layout test in
-// tests/e2e/run.js, which measures the flash line and goes red on a shift.
-const DUST_THRESHOLD_MESSAGE =
-    "Please enter a whole number of gwei, zero or greater.";
 
 // Returns the threshold in gwei, or null if the input is not one.
 function parseDustThresholdGwei(raw) {

src/popup/index.html

@@ -33,7 +33,7 @@
             <!-- ============ FLASH MESSAGE AREA ============ -->
             <div
                 id="flash-msg"
-                class="text-xs text-muted min-h-[1.25rem] mb-1"
+                class="text-xs text-muted min-h-[1.25rem] mb-1 truncate"
             ></div>
 
             <!-- ============ WELCOME / FIRST USE ============ -->

src/popup/views/addToken.js

@@ -28,9 +28,7 @@ function init(ctx) {
     $("btn-add-token-confirm").addEventListener("click", async () => {
         const contractAddr = $("add-token-address").value.trim();
         if (!contractAddr || !contractAddr.startsWith("0x")) {
-            showFlash(
+            showFlash("Enter a valid contract address starting with 0x.");
-                "Please enter a valid contract address starting with 0x.",
-            );
             return;
         }
         const already = state.trackedTokens.find(
@@ -71,8 +69,15 @@ function init(ctx) {
             require("./addressDetail").show();
         } catch (e) {
             const detail = e.shortMessage || e.message || String(e);
-            log.errorf("Token lookup failed for", contractAddr, detail);
+            log.errorf("Adding token failed for", contractAddr, detail);
-            showFlash(detail);
+            // lookupTokenInfo() rejects a contract with a one-line message
+            // starting "Not a valid ERC-20 token". Any other error, such as a
+            // failed save, can be far longer, so it is only logged.
+            showFlash(
+                detail.startsWith("Not a valid ERC-20 token")
+                    ? detail
+                    : "Could not add the token.",
+            );
             infoEl.textContent = "";
             infoEl.style.visibility = "hidden";
         }

src/popup/views/addWallet.js

@@ -142,15 +142,13 @@ function validatePassword() {
 async function importMnemonic(ctx) {
     const mnemonic = $("wallet-mnemonic").value.trim();
     if (!mnemonic) {
-        showFlash("Enter a recovery phrase or press the die to generate one.");
+        showFlash("Enter a recovery phrase, or press the die.");
         return;
     }
     const words = mnemonic.split(/\s+/);
     if (words.length !== 12 && words.length !== 24) {
         showFlash(
-            "Recovery phrase must be 12 or 24 words. You entered " +
+            "Recovery phrase must be 12 or 24 words, not " + words.length + ".",
-                words.length +
-                ".",
         );
         return;
     }
@@ -163,14 +161,12 @@ async function importMnemonic(ctx) {
     const { xpub, firstAddress } = hdWalletFromMnemonic(mnemonic);
     const xpubDup = findWalletByXpub(xpub);
     if (xpubDup) {
-        showFlash(
+        showFlash("This recovery phrase is already added.");
-            "This recovery phrase is already added (" + xpubDup.name + ").",
-        );
         return;
     }
     const addrDup = findWalletByAddress(firstAddress);
     if (addrDup) {
-        showFlash("Address already exists in wallet (" + addrDup.name + ").");
+        showFlash("Address already exists in a wallet.");
         return;
     }
     const encrypted = await encryptWithPassword(mnemonic, pw);
@@ -229,9 +225,7 @@ async function importPrivateKey(ctx) {
     if (!pw) return;
     const duplicate = findWalletByAddress(addr);
     if (duplicate) {
-        showFlash(
+        showFlash("This address already exists in a wallet.");
-            "This address already exists in wallet (" + duplicate.name + ").",
-        );
         return;
     }
     const encrypted = await encryptWithPassword(key, pw);
@@ -258,36 +252,29 @@ async function importXprvKey(ctx) {
         return;
     }
     if (!isValidXprv(xprv)) {
-        showFlash(
+        showFlash("That extended private key is not valid.");
-            "That extended private key is not valid. Please check it and try again.",
-        );
         return;
     }
     if (!isMasterExtendedKey(xprv)) {
-        showFlash(
+        showFlash("Please paste the master key, not a child key.");
-            "That is an account-level or child key, which cannot be imported. " +
-                "Please paste the master extended private key for the wallet.",
-        );
         return;
     }
     let result;
     try {
         result = hdWalletFromXprv(xprv);
     } catch {
-        showFlash(
+        showFlash("That extended private key is not valid.");
-            "That extended private key is not valid. Please check it and try again.",
-        );
         return;
     }
     const { xpub, firstAddress } = result;
     const xpubDup = findWalletByXpub(xpub);
     if (xpubDup) {
-        showFlash("This key is already added (" + xpubDup.name + ").");
+        showFlash("This key is already added.");
         return;
     }
     const addrDup = findWalletByAddress(firstAddress);
     if (addrDup) {
-        showFlash("Address already exists in wallet (" + addrDup.name + ").");
+        showFlash("Address already exists in a wallet.");
         return;
     }
     const pw = validatePassword();

src/popup/views/helpers.js

@@ -223,15 +223,19 @@ function clearFlash() {
         flashTimer = null;
     }
     $("flash-msg").textContent = "";
+    $("flash-msg").title = "";
 }
 
+// The flash line reserves exactly one line, and a message that wrapped would
+// push the screen below it down (README, No Layout Shift). So #flash-msg never
+// wraps: text too long for the line is cut with an ellipsis, and the whole
+// message is also put in the line's title. Write messages to fit, at most 50
+// characters, so none is cut.
 function showFlash(msg, duration = 2000) {
     clearFlash();
     $("flash-msg").textContent = msg;
-    flashTimer = setTimeout(() => {
+    $("flash-msg").title = msg;
-        $("flash-msg").textContent = "";
+    flashTimer = setTimeout(clearFlash, duration);
-        flashTimer = null;
-    }, duration);
 }
 
 // A stored token balance as a number, or null when there is no number in it.

src/popup/views/send.js

@@ -63,13 +63,13 @@ function validateToAddress(value) {
             if (checksummed !== v) {
                 return {
                     valid: false,
-                    error: "Address checksum is invalid. Please double-check the address.",
+                    error: "Address checksum is invalid. Check the address.",
                 };
             }
         } catch {
             return {
                 valid: false,
-                error: "Address checksum is invalid. Please double-check the address.",
+                error: "Address checksum is invalid. Check the address.",
             };
         }
     }
@@ -211,7 +211,7 @@ function init(_ctx) {
                 const provider = getProvider(state.rpcUrl, state.networkId);
                 const resolved = await provider.resolveName(to);
                 if (!resolved) {
-                    showFlash("Could not resolve " + to);
+                    showFlash("That ENS name has no address.");
                     return;
                 }
                 resolvedTo = resolved;

src/popup/views/settings.js

@@ -264,18 +264,12 @@ function init(ctx) {
             const json = await resp.json();
             if (json.error) {
                 log.errorf("RPC validation error:", json.error);
-                showFlash("Endpoint returned error: " + json.error.message);
+                showFlash("Endpoint returned an error.");
                 return;
             }
             const net = currentNetwork();
             if (json.result !== net.chainId) {
-                showFlash(
+                showFlash("Wrong network: expected " + net.name + ".");
-                    "Wrong network (expected " +
-                        net.name +
-                        ", got chain " +
-                        json.result +
-                        ").",
-                );
                 return;
             }
         } catch (e) {

src/popup/views/settingsAddToken.js

@@ -115,9 +115,7 @@ function init(_ctx) {
     $("btn-settings-addtoken-manual").addEventListener("click", async () => {
         const addr = $("settings-addtoken-address").value.trim();
         if (!addr || !addr.startsWith("0x")) {
-            showFlash(
+            showFlash("Enter a valid contract address starting with 0x.");
-                "Please enter a valid contract address starting with 0x.",
-            );
             return;
         }
         if (isTracked(addr)) {
@@ -155,8 +153,15 @@ function init(_ctx) {
             ctx.doRefreshAndRender();
         } catch (e) {
             const detail = e.shortMessage || e.message || String(e);
-            log.errorf("Token lookup failed for", addr, detail);
+            log.errorf("Adding token failed for", addr, detail);
-            showFlash(detail);
+            // lookupTokenInfo() rejects a contract with a one-line message
+            // starting "Not a valid ERC-20 token". Any other error, such as a
+            // failed save, can be far longer, so it is only logged.
+            showFlash(
+                detail.startsWith("Not a valid ERC-20 token")
+                    ? detail
+                    : "Could not add the token.",
+            );
             infoEl.textContent = "";
             infoEl.style.visibility = "hidden";
         }

src/shared/walletDefects.js

@@ -41,12 +41,10 @@ const DEFECTS = {
                 "changed or removed, and this wallet stays until you delete " +
                 "it yourself.",
         ],
-        // One sentence for the places that have room for one: the flash on a
+        // One line, for the flash on a blocked Send and the inline error on
-        // blocked Send, the inline error on the approval screens.
+        // the approval screens. It must fit on the flash line; see showFlash()
-        shortMessage:
+        // in src/popup/views/helpers.js.
-            "This wallet cannot sign, because it was imported from an " +
+        shortMessage: "This wallet cannot sign. See the wallet list.",
-            "extended private key that is not a master key. The wallet list " +
-            "explains what happened.",
     },
 };
 

tests/dustThreshold.test.js

@@ -99,12 +99,13 @@ describe("the flash line the message is shown in", () => {
     // length, including one that wrapped to two lines and pushed the
     // settings view down 12px.
     //
-    // The assertion that actually measures — empty line vs. the message,
+    // The line cuts a message too long for it with an ellipsis (see
-    // real Chromium, documented 360x600 popup — is
+    // showFlash() in src/popup/views/helpers.js). The assertions that
-    // "a rejected dust threshold shifts no layout (#233)" in
+    // measure that, in a real browser at the documented 360x600 popup, are
-    // tests/e2e/run.js, run by make test-e2e. It is not in make check
+    // "a rejected dust threshold shifts no layout (#233)" and "an over-long
-    // because REPO_POLICIES.md caps make test at 20 seconds and a browser
+    // flash message keeps to one line (#252)" in tests/e2e/run.js, run by
-    // suite does not fit; run it before changing the wording.
+    // make test-e2e. They are not in make check because REPO_POLICIES.md
+    // caps make test at 20 seconds and a browser suite does not fit.
     test("reserves its height in the markup", () => {
         const flashLine = POPUP_HTML.match(
             /<div\s+id="flash-msg"\s+class="([^"]*)"/,

tests/e2e/run.js

@@ -1320,17 +1320,13 @@ async function waitForFilledFlashLine(page) {
 }
 
 // README, No Layout Shift: the rejection message goes into #flash-msg,
-// whose min-h-[1.25rem] reserves exactly ONE line at text-xs. Reserving
+// whose min-h-[1.25rem] reserves exactly ONE line at text-xs, and which
-// the space is not enough on its own — a message too long for one line
+// cuts a message too long for that line with an ellipsis rather than wrap
-// wraps and pushes everything below it down anyway, which is what the
+// it. This shows the real message and measures that nothing moves; the
-// first version of this change shipped: 75 characters, 32px, the settings
+// test after it does the same with a message several lines long. Both
-// view and the threshold field 12px lower than with an empty line.
+// measure rather than inspect markup: the unit suite runs on the node
-//
+// environment with no layout engine, where every height is zero (see the
-// So this measures rather than inspects markup. It is the only assertion
+// note in tests/dustThreshold.test.js).
-// in the repo that can see the wording grow: the unit suite runs on the
-// node environment with no layout engine, where every height is zero (see
-// the note in tests/dustThreshold.test.js). Lengthen
-// DUST_THRESHOLD_MESSAGE past one line and this test goes red.
 test("a rejected dust threshold shifts no layout (#233)", async (env) => {
     const page = await openPopup(env.ctx, env.popupUrl);
     try {
@@ -1377,11 +1373,11 @@ test("a rejected dust threshold shifts no layout (#233)", async (env) => {
         );
         assert(
             after.flashHeight === before.flashHeight,
-            "the message does not fit the reserved line: " +
+            "the message does not keep to the reserved line: " +
                 before.flashHeight +
                 "px empty vs " +
                 after.flashHeight +
-                "px with the message. Shorten DUST_THRESHOLD_MESSAGE",
+                "px with the message",
         );
         assert(
             after.settingsTop === before.settingsTop,
@@ -1400,6 +1396,59 @@ test("a rejected dust threshold shifts no layout (#233)", async (env) => {
     }
 });
 
+// ------------------------------------------------ the flash line (#252)
+
+// #flash-msg never wraps: a message too long for its one line is cut with an
+// ellipsis (see showFlash() in src/popup/views/helpers.js). This puts a
+// message several lines long into it and measures that the line and the
+// screen below it stay where they were.
+test("an over-long flash message keeps to one line (#252)", async (env) => {
+    const page = await openPopup(env.ctx, env.popupUrl);
+    try {
+        await page.setViewportSize(POPUP_VIEWPORT);
+        await openSettings(page);
+
+        const before = await page.evaluate(measureFlashLine);
+        const overflows = await page.evaluate(() => {
+            const line = document.getElementById("flash-msg");
+            line.textContent =
+                "This message is far too long for one line. ".repeat(5);
+            return line.scrollWidth > line.clientWidth;
+        });
+        const after = await page.evaluate(measureFlashLine);
+
+        assert(
+            after.flashHeight === before.flashHeight,
+            "the flash line is " +
+                before.flashHeight +
+                "px before and " +
+                after.flashHeight +
+                "px with an over-long message, so it wraps",
+        );
+        assert(
+            after.settingsTop === before.settingsTop,
+            "the settings view moved " +
+                (after.settingsTop - before.settingsTop) +
+                "px when the message appeared",
+        );
+        assert(
+            after.fieldTop === before.fieldTop,
+            "the dust threshold field moved " +
+                (after.fieldTop - before.fieldTop) +
+                "px when the message appeared",
+        );
+        // Checked last: a line that wraps does not run past its right edge,
+        // so this only shows the message really was cut once nothing moved.
+        assert(
+            overflows,
+            "the message fits on the line, so it proves nothing: " +
+                JSON.stringify(after.text),
+        );
+    } finally {
+        await page.close();
+    }
+});
+
 // --------------------------------------------- confirmation screen (#238)
 //
 // The screen that decides what gets signed. The arithmetic underneath it

tests/flashLine.test.js

@@ -0,0 +1,130 @@
+// The flash line (#252). #flash-msg reserves one line and cuts a message too
+// long for it with an ellipsis; that is measured in a real browser by
+// tests/e2e/run.js. Here: showFlash() keeps the whole message readable in the
+// line's title, and the two add-token screens flash a fixed line, not the text
+// of whatever error adding the token threw.
+
+const ADDRESS = "0x1111111111111111111111111111111111111111";
+
+let elements;
+
+function fakeElement() {
+    return {
+        value: "",
+        textContent: "",
+        title: "",
+        style: {},
+        listeners: {},
+        addEventListener(event, handler) {
+            this.listeners[event] = handler;
+        },
+    };
+}
+
+// Stands in for document.getElementById(): one fake element per id.
+function element(id) {
+    return (elements[id] ||= fakeElement());
+}
+
+beforeEach(() => {
+    jest.resetModules();
+    elements = {};
+    globalThis.document = { getElementById: element };
+    // state.js reads chrome.storage.local at load.
+    globalThis.chrome = {
+        storage: { local: { get: async () => ({}), set: async () => {} } },
+    };
+});
+
+afterEach(() => {
+    jest.dontMock("../src/popup/views/helpers");
+    jest.dontMock("../src/shared/state");
+    jest.dontMock("../src/shared/balances");
+    jest.restoreAllMocks();
+    jest.useRealTimers();
+    delete globalThis.document;
+    delete globalThis.chrome;
+});
+
+test("showFlash() puts the whole message in the title, and clears both", () => {
+    jest.useFakeTimers();
+    const { showFlash } = require("../src/popup/views/helpers");
+
+    showFlash("Saved.");
+    expect(element("flash-msg").textContent).toBe("Saved.");
+    expect(element("flash-msg").title).toBe("Saved.");
+
+    jest.advanceTimersByTime(2000);
+    expect(element("flash-msg").textContent).toBe("");
+    expect(element("flash-msg").title).toBe("");
+});
+
+describe.each([
+    ["addToken", "add-token-address", "btn-add-token-confirm"],
+    [
+        "settingsAddToken",
+        "settings-addtoken-address",
+        "btn-settings-addtoken-manual",
+    ],
+])("adding a token on %s", (view, field, button) => {
+    let flashes;
+    let errors;
+
+    // Clicks the screen's add button with lookupTokenInfo() and saveState()
+    // replaced by the given functions.
+    async function add(lookupTokenInfo, saveState) {
+        flashes = [];
+        errors = jest.spyOn(console, "error").mockImplementation(() => {});
+        jest.spyOn(console, "log").mockImplementation(() => {});
+        jest.doMock("../src/shared/balances", () => ({ lookupTokenInfo }));
+        jest.doMock("../src/shared/state", () => ({
+            state: { trackedTokens: [] },
+            saveState,
+        }));
+        jest.doMock("../src/popup/views/helpers", () => ({
+            $: element,
+            showView: () => {},
+            showFlash: (msg) => flashes.push(msg),
+            escapeHtml: (s) => s,
+            goBack: () => {},
+        }));
+
+        require("../src/popup/views/" + view).init({
+            doRefreshAndRender: () => {},
+        });
+        element(field).value = ADDRESS;
+        await element(button).listeners.click();
+    }
+
+    test("a failed save flashes a fixed line and logs the error", async () => {
+        const detail = "A sentence about the stored record. ".repeat(4);
+
+        await add(
+            async () => ({ symbol: "TKN", decimals: 18, name: "Token" }),
+            async () => {
+                throw new Error(detail);
+            },
+        );
+
+        expect(flashes).toEqual(["Could not add the token."]);
+        expect(errors).toHaveBeenCalledWith(
+            "[AutistMask]",
+            "Adding token failed for",
+            ADDRESS,
+            detail,
+        );
+    });
+
+    test("a contract that is not a token flashes the lookup message", async () => {
+        const detail = "Not a valid ERC-20 token (symbol() failed).";
+
+        await add(
+            async () => {
+                throw new Error(detail);
+            },
+            async () => {},
+        );
+
+        expect(flashes).toEqual([detail]);
+    });
+});

tests/inpageErrors.test.js

@@ -49,11 +49,12 @@ class StubCustomEvent extends StubEvent {
     }
 }
 
-// Every code the background emits on the RPC path today, read out of
+// Examples of codes the background emits on the RPC path, read out of
-// src/background/index.js. The provider must not know this list — it passes
+// src/background/index.js. The provider must not know any list of codes — it
-// through whatever arrived — but the cases below are the real ones.
+// passes through whatever arrived — but the cases below are real ones.
 const REJECTED = 4001; // user rejected the request
 const UNAUTHORIZED = 4100; // site not connected / wrong address
+const UNSUPPORTED_METHOD = 4200; // a method the wallet does not implement
 const UNRECOGNIZED_CHAIN = 4902; // switch/add to an unsupported chain
 
 // A stub window with the four things inpage.js touches: message listeners,
@@ -115,6 +116,41 @@ async function rejectionFrom(start, response) {
     return outcome.error;
 }
 
+// The reply the real background worker (src/background/index.js) sends for
+// `method`, loaded against just enough of the extension API to receive one
+// RPC message. Same shape as tests/coldWorkerChainId.test.js.
+function backgroundReply(method) {
+    jest.resetModules();
+    jest.doMock("../src/shared/alarms", () => ({
+        BALANCE_REFRESH_ALARM: "balance",
+        BALANCE_REFRESH_PERIOD_MINUTES: 1,
+        ensureRecurringAlarms: async () => {},
+        registerAlarmHandlers: () => {},
+    }));
+
+    let messageListener = null;
+    global.chrome = {
+        runtime: {
+            onMessage: {
+                addListener: (fn) => {
+                    messageListener = fn;
+                },
+            },
+            onConnect: { addListener: () => {} },
+        },
+    };
+
+    require("../src/background/index");
+
+    return new Promise((resolve) => {
+        messageListener(
+            { type: "AUTISTMASK_RPC", method, params: [] },
+            { origin: "https://dapp.example" },
+            resolve,
+        );
+    });
+}
+
 describe("an EIP-1193 code reaches the page", () => {
     test("a user rejection arrives as code 4001", async () => {
         const err = await rejectionFrom(
@@ -164,8 +200,9 @@ describe("an EIP-1193 code reaches the page", () => {
         expect(err.message).toBe(message);
     });
 
-    // The provider is not allowed to know the list above: a code added to the
+    // The provider is not allowed to know the codes above: any other code,
-    // background later must reach the page without this file being edited.
+    // including one added to the background later, must reach the page
+    // without inpage.js being edited.
     test("a code the provider has never heard of is passed through", async () => {
         const err = await rejectionFrom(
             (p) => p.request({ method: "eth_accounts" }),
@@ -203,6 +240,26 @@ describe("an EIP-1193 code reaches the page", () => {
     });
 });
 
+// The reply here is the background's own, not one written in this file: it
+// used to carry no code for a method the wallet does not implement
+// (https://git.eeqj.de/sneak/AutistMask/issues/279), so a site probing for an
+// optional method could not tell "not implemented" from "the call failed".
+describe("a method the wallet does not implement", () => {
+    afterEach(() => {
+        delete global.chrome;
+    });
+
+    test("reaches the page as code 4200", async () => {
+        const method = "wallet_noSuchMethod";
+        const err = await rejectionFrom(
+            (p) => p.request({ method }),
+            await backgroundReply(method),
+        );
+        expect(err.code).toBe(UNSUPPORTED_METHOD);
+        expect(err.message).toBe("Unsupported method: " + method);
+    });
+});
+
 describe("the message is untouched", () => {
     test("a coded error keeps the message byte for byte", async () => {
         const message =