diff --git a/tests/popupBoot.test.js b/tests/popupBoot.test.js
index bbc0067..5980cbf 100644
--- a/tests/popupBoot.test.js
+++ b/tests/popupBoot.test.js
@@ -9,6 +9,7 @@ const {
     bootPopup,
     cleanupPopup,
     unversionedValidProfile,
+    ADDRESS,
 } = require("./support/popupBoot");
 
 const FLASH_MS = 10;
@@ -40,3 +41,47 @@ test("a flash message's timer does not fire after cleanupPopup()", async () => {
 
     await wait(FLASH_MS * 5);
 });
+
+// A boot onto a screen whose requests no mocked module answers: they reach the
+// boot's fetch() stub, never the internet
+// (https://git.eeqj.de/sneak/AutistMask/issues/559).
+function restoringOnto(view, viewData) {
+    return unversionedValidProfile({
+        currentView: view,
+        selectedWallet: 0,
+        selectedAddress: 0,
+        viewStack: ["main"],
+        viewData,
+    });
+}
+
+test("the transaction screen's Blockscout request reaches the stub", async () => {
+    const page = await bootPopup(
+        restoringOnto("transaction", {
+            tx: { hash: "0x1", from: ADDRESS, to: ADDRESS },
+        }),
+    );
+
+    expect(page.visibleViews()).toEqual(["transaction"]);
+    expect(globalThis.fetch.mock.calls.map((call) => call[0])).toEqual([
+        "https://eth.blockscout.com/api/v2/transactions/0x1",
+    ]);
+});
+
+test("the confirmation screen's Etherscan lookup reaches the stub", async () => {
+    const page = await bootPopup(
+        restoringOnto("confirm-tx", {
+            pendingTx: {
+                token: "ETH",
+                from: ADDRESS,
+                to: ADDRESS,
+                amount: "1",
+            },
+        }),
+    );
+
+    expect(page.visibleViews()).toEqual(["confirm-tx"]);
+    expect(globalThis.fetch.mock.calls.map((call) => call[0])).toEqual([
+        "https://etherscan.io/address/" + ADDRESS,
+    ]);
+});
diff --git a/tests/support/popupBoot.js b/tests/support/popupBoot.js
index 02f98d0..b2c6c6b 100644
--- a/tests/support/popupBoot.js
+++ b/tests/support/popupBoot.js
@@ -45,6 +45,7 @@ const realSetInterval = globalThis.setInterval;
 const realClearInterval = globalThis.clearInterval;
 const realSetTimeout = globalThis.setTimeout;
 const realClearTimeout = globalThis.clearTimeout;
+const realFetch = globalThis.fetch;
 
 // Every timeout set since the last cleanupPopup() that has neither fired nor
 // been cleared. cleanupPopup() clears them all.
@@ -260,8 +261,10 @@ async function settle() {
 async function bootPopup(stored, options) {
     jest.resetModules();
 
-    // The three modules that reach the network. None is on the path under
-    // test; all would make the suite hit the internet.
+    // Three of the modules that reach the network. None is on the path under
+    // test; all would make the suite hit the internet. Every other request,
+    // such as the transaction screen's Blockscout lookup and the Etherscan
+    // lookup in src/shared/etherscanLabels.js, meets the fetch() stub below.
     jest.doMock("../../src/shared/prices", () => ({
         prices: {},
         refreshPrices: jest.fn(async () => {}),
@@ -334,6 +337,12 @@ async function bootPopup(stored, options) {
             windowListeners[name].push(fn);
         },
     };
+    // Every fetch() the page makes fails, as it does elsewhere in the suite,
+    // instead of reaching the internet. A new stub per boot, so its calls are
+    // that boot's requests. Put back by cleanupPopup().
+    globalThis.fetch = jest.fn(() => {
+        throw new Error("tests must not perform network requests");
+    });
     // The ten-second refresh init() starts, and a transaction wait's timers,
     // would outlive the test. So every interval is recorded rather than
     // started, clearInterval() removes it as a browser would, and tick() below
@@ -435,6 +444,7 @@ function cleanupPopup() {
     pendingTimeouts.clear();
     globalThis.setTimeout = realSetTimeout;
     globalThis.clearTimeout = realClearTimeout;
+    globalThis.fetch = realFetch;
 }
 
 module.exports = {