fix: render the view "Back" lands on after the popup is reopened (closes #268)

goBack() only unhid its target. A reopened popup renders the wallet list
and the one view restoreView() lands on, so every other view is still the
blank static template from index.html: pressing Back from Settings onto an
address showed an empty address line and no balances, and the same held for
address-token, receive, confirm-tx and transaction.

The per-view dispatch and its data guards move out of restoreView() into
src/popup/viewRouter.js, and goBack() now routes a popped view through the
same code by way of a renderer index.js registers with setBackRenderer().
A view whose backing state is gone falls back to Home the way the restore
does, rather than showing an empty template.

The Back path renders only a view this page load has not rendered yet.
viewRouter.js keeps a page-load-scoped set of rendered views, written by
showView() — the last thing every render path runs, forward navigation and
the restore alike, so a view added later registers itself rather than
needing to be remembered. Back onto a view already in that set unhides it
and nothing more, exactly as it does for a view outside RESTORABLE_VIEWS:
rendering a second time would re-fetch and overwrite what the view holds,
such as an edit typed into Settings and not yet saved. Home stays the
exception and is re-rendered on every Back, as it was when goBack() called
the renderWalletList() registered through setRenderMain(), so the wallet
list reflects what changed while the user was away from it.

tests/backNavigation.test.js drives the real goBack() over the reproduction
and over each of address-token, receive, confirm-tx, transaction, success-tx
and error-tx, with and without their backing state, and pins that a view
this page load already rendered — a live-session view, or Settings revisited
— is only unhidden, while Home still re-renders. tests/e2e/run.js adds three
cases against the real popup in a real browser, because make check cannot
see a blank view: a real close and reopen then Back onto the address screen
and onto Receive, and an in-session Back onto Settings that must keep
unsaved input. All three were demonstrated failing first.

tests/e2e/run.js

@@ -419,6 +419,172 @@ test("reopening the popup never lands on the phrase screen (#161)", async (env)
     assertWiped(st, env.phrase, "after reopening the popup");
 });
 
+// ------------------------------- Back after reopening the popup (#268)
+
+// A reopened popup renders the wallet list and the view it restores onto,
+// and nothing else: every other screen is still the blank static template
+// from index.html. Back used to only unhide its target, which is why these
+// have to run against the real popup — the template is present and
+// well-formed, so only its emptiness distinguishes the defect, and only a
+// real reopen produces it.
+
+// Everything the address screen must have on it, read out of the DOM.
+function addressScreenState(page) {
+    return page.evaluate(() => {
+        const line = document.getElementById("address-line");
+        const balances = document.getElementById("address-balances");
+        return {
+            hidden: document
+                .getElementById("view-address")
+                .classList.contains("hidden"),
+            line: line ? line.innerText.trim() : "",
+            balances: balances ? balances.innerText.trim() : "",
+        };
+    });
+}
+
+// Close and reopen the page rather than reload it: that is what the toolbar
+// popup does, and it is the only thing that produces the unrendered views.
+async function reopenPopup(env, restoredView) {
+    await env.page.close();
+    env.page = await openPopup(env.ctx, env.popupUrl);
+    await visible(env.page, restoredView);
+}
+
+// The reproduction from the issue, step for step.
+test("Back after reopening the popup renders the address screen (#268)", async (env) => {
+    await openAddressDetail(env.page);
+    const before = await addressScreenState(env.page);
+    assert(
+        before.line.length > 0,
+        "the address screen was blank to begin with",
+    );
+
+    await env.page.click("#btn-settings");
+    await visible(env.page, "#view-settings");
+
+    await reopenPopup(env, "#view-settings");
+
+    await env.page.click("#btn-settings-back");
+    await visible(env.page, "#view-address");
+
+    const after = await addressScreenState(env.page);
+    assert(
+        after.line === before.line,
+        "the address line reads " +
+            JSON.stringify(after.line) +
+            ", expected " +
+            JSON.stringify(before.line),
+    );
+    assert(
+        after.balances.includes("ETH"),
+        "the balances read " + JSON.stringify(after.balances),
+    );
+});
+
+// The same defect one screen further in. Receive holds the address twice
+// over — as text and as the QR code the sender scans — and a blank one is
+// worse than a missing screen.
+// Everything the Receive screen must have on it. The QR code is read as
+// pixels, not as an element: the blank template carries the canvas too, a
+// default 300x150 one with nothing drawn on it and every pixel fully
+// transparent. A drawn QR paints an opaque background across the whole
+// canvas, so a single opaque pixel is the whole question.
+function receiveScreenState(page) {
+    return page.evaluate(() => {
+        const block = document.getElementById("receive-address-block");
+        const canvas = document.getElementById("receive-qr");
+        const px = canvas
+            .getContext("2d")
+            .getImageData(0, 0, canvas.width, canvas.height).data;
+        let opaque = 0;
+        for (let i = 3; i < px.length; i += 4) {
+            if (px[i] > 0) opaque += 1;
+        }
+        return {
+            address: block.dataset.full || "",
+            text: block.innerText.trim(),
+            qrOpaquePixels: opaque,
+        };
+    });
+}
+
+test("Back after reopening the popup renders the Receive screen (#268)", async (env) => {
+    await openAddressDetail(env.page);
+    await env.page.click("#btn-receive");
+    await visible(env.page, "#view-receive");
+    const before = await receiveScreenState(env.page);
+    assert(
+        /^0x[0-9a-fA-F]{40}$/.test(before.address),
+        "Receive showed no address to begin with: " +
+            JSON.stringify(before.address),
+    );
+
+    await env.page.click("#btn-settings");
+    await visible(env.page, "#view-settings");
+
+    await reopenPopup(env, "#view-settings");
+
+    await env.page.click("#btn-settings-back");
+    await visible(env.page, "#view-receive");
+
+    const shown = await receiveScreenState(env.page);
+    assert(
+        shown.address === before.address,
+        "Receive shows " +
+            JSON.stringify(shown.address) +
+            ", expected " +
+            JSON.stringify(before.address),
+    );
+    assert(
+        shown.text.includes(before.address),
+        "the Receive address is not on screen: " + JSON.stringify(shown.text),
+    );
+    assert(shown.qrOpaquePixels > 0, "Receive shows an unpainted QR code");
+
+    // Leave the suite where it found it.
+    await env.page.click("#btn-receive-back");
+    await visible(env.page, "#view-address");
+    await env.page.click("#btn-address-back");
+    await visible(env.page, "#view-main");
+});
+
+// The other half of the requirement: Back renders a screen this page load
+// never rendered, and must NOT re-render one it already has on screen.
+// settings.show() reassigns #settings-rpc from persisted state, so
+// re-rendering Settings on the way back would silently revert whatever the
+// user typed and had not saved yet — and they could then press Save and
+// store the value they believed they had replaced. No reopen here: this is
+// an ordinary in-session forward-and-back, which is exactly why the render
+// must not happen.
+test("Back onto Settings keeps unsaved input (#268)", async (env) => {
+    await visible(env.page, "#view-main");
+    await env.page.click("#btn-settings");
+    await visible(env.page, "#view-settings");
+
+    const typed = "https://rpc.example.invalid/unsaved";
+    await env.page.fill("#settings-rpc", typed);
+
+    await env.page.click("#btn-settings-add-token");
+    await visible(env.page, "#view-settings-addtoken");
+    await env.page.click("#btn-settings-addtoken-back");
+    await visible(env.page, "#view-settings");
+
+    const kept = await env.page.inputValue("#settings-rpc");
+    assert(
+        kept === typed,
+        "the unsaved RPC URL reads " +
+            JSON.stringify(kept) +
+            ", expected " +
+            JSON.stringify(typed),
+    );
+
+    // Leave the suite where it found it. The typed value was never saved,
+    // and Settings reloads the field from state next time it renders.
+    await env.page.click("#btn-settings-back");
+    await visible(env.page, "#view-main");
+});
+
 // -------------------------------------------- address removal (#162)
 
 // Number of address rows across every wallet in the list, counted in the DOM