test: drive the Settings screen in a browser and guard every popup element id (closes #229)

Nothing exercised the Settings view in a browser, and jest runs in the
node environment with no DOM, so the densest run of $("...") lookups in
the codebase was unverified at runtime. A wrong id is valid JavaScript
naming a defined function: $() returns null and the next property access
throws, which inside a view's init() aborts the rest of the popup's
init() and leaves every screen blank.

Two halves, because they catch different things.

The e2e suite (tests/e2e/run.js) gains six cases between the address
removal and dust threshold sections. They assert the About well and the
wallet list were actually written — show() populates those last, so
reading them back proves the whole of show() ran rather than just enough
of it to unhide the section — that the four Token Spam Protection
controls are real input[type=checkbox] elements defaulted on, and that
the theme and network selectors offer exactly the choices
src/shared/networks.js and index.html define while carrying the
persisted value. One filter is then toggled off and back on across a
popup reopen each way, which runs the change handler, saveState(),
loadState() and the init() assignment rather than only looking at the
screen. Each group records a coverage key and a final case demands the
exact set, so a section that silently stopped running reddens the suite
instead of shrinking it.

tests/popupElementIds.test.js is the general half and needs no browser,
so jest picks it up and it runs in make check: every literal id reached
through $(), document.getElementById(), showError()/hideError() and
showView() must exist in src/popup/index.html, no id in index.html may
be defined twice, and the scan asserts it found the code and the markup
so it cannot pass by covering nothing. Only literal arguments are
resolvable statically; $(containerId) and a lookup naming the wrong
existing element are the browser suites' job, and README says so.

Demonstrated against three deliberate breaks. A typo'd id in
settings.js reddens both halves, the e2e run reporting
"pageerror: Cannot set properties of null (setting 'checked')" against
its first test. A handler bound to the wrong but existing element passes
the static guard and reddens only the new functional case. A typo in a
view no browser suite opens reddens only the static guard.

tests/popupElementIds.test.js

@@ -0,0 +1,182 @@
+// Every element id the popup views look up must exist in the markup they
+// look it up in.
+//
+// The failure this catches: `$("settings-hide-dsut")` is valid JavaScript
+// referring to a defined function, so neither jest (node environment, no
+// DOM) nor a linter has anything to object to. At runtime `$()` returns
+// null and the next property access throws, which in `init()` aborts the
+// rest of that view's wiring and takes the whole screen down. Settings is
+// the densest concentration of these lookups in the codebase.
+//
+// This is the cheap general half of the guard: it runs in `make check`
+// with no browser and covers every id in every view, not the ones some
+// test happens to click. The expensive specific half is the Settings
+// section of the end-to-end suite (tests/e2e/run.js), which proves the
+// screen actually comes up and its controls work.
+//
+// Scope and limits, stated rather than implied:
+//   - Only literal string arguments are resolvable statically. A call
+//     like `$(containerId)` is invisible here; those are covered by the
+//     e2e run instead.
+//   - `document.getElementById()` is checked too, minus the ids listed in
+//     RUNTIME_CREATED_IDS, which name nodes the code creates itself and
+//     which are legitimately absent from the static markup.
+
+"use strict";
+
+const fs = require("fs");
+const path = require("path");
+
+const POPUP_DIR = path.join(__dirname, "..", "src", "popup");
+const POPUP_HTML_PATH = path.join(POPUP_DIR, "index.html");
+
+// Nodes built at runtime rather than authored in index.html. Each one must
+// be created unconditionally by the code before it is ever looked up.
+const RUNTIME_CREATED_IDS = new Set([
+    // Created by updateDebugBanner() in src/popup/views/helpers.js.
+    "debug-banner",
+]);
+
+// Every id lookup the popup performs with a literal argument, as
+// {id, file, line, source} records.
+//
+// showView("x") is included because it resolves to the element id
+// "view-x": a view name with no matching section is the same defect one
+// indirection further out.
+const PATTERNS = [
+    { re: /\$\(\s*"([^"\n]+)"\s*\)/g, id: (m) => m[1], source: "$()" },
+    {
+        re: /document\.getElementById\(\s*"([^"\n]+)"\s*\)/g,
+        id: (m) => m[1],
+        source: "getElementById()",
+    },
+    {
+        re: /\b(?:showError|hideError)\(\s*"([^"\n]+)"/g,
+        id: (m) => m[1],
+        source: "showError()/hideError()",
+    },
+    {
+        re: /\bshowView\(\s*"([^"\n]+)"\s*\)/g,
+        id: (m) => "view-" + m[1],
+        source: "showView()",
+    },
+];
+
+function jsFilesUnder(dir) {
+    const out = [];
+    for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+        const full = path.join(dir, entry.name);
+        if (entry.isDirectory()) {
+            out.push(...jsFilesUnder(full));
+        } else if (entry.name.endsWith(".js")) {
+            out.push(full);
+        }
+    }
+    return out.sort();
+}
+
+function lineOf(text, index) {
+    return text.slice(0, index).split("\n").length;
+}
+
+function collectReferences() {
+    const refs = [];
+    for (const file of jsFilesUnder(POPUP_DIR)) {
+        const text = fs.readFileSync(file, "utf8");
+        const rel = path.relative(path.join(__dirname, ".."), file);
+        for (const { re, id, source } of PATTERNS) {
+            re.lastIndex = 0;
+            let m;
+            while ((m = re.exec(text)) !== null) {
+                refs.push({
+                    id: id(m),
+                    file: rel,
+                    line: lineOf(text, m.index),
+                    source,
+                });
+            }
+        }
+    }
+    return refs;
+}
+
+function collectHtmlIds(html) {
+    const ids = [];
+    const re = /\bid="([^"]+)"/g;
+    let m;
+    while ((m = re.exec(html)) !== null) ids.push(m[1]);
+    return ids;
+}
+
+const HTML = fs.readFileSync(POPUP_HTML_PATH, "utf8");
+const HTML_IDS = collectHtmlIds(HTML);
+const HTML_ID_SET = new Set(HTML_IDS);
+const REFERENCES = collectReferences();
+
+describe("every element id the popup looks up exists in its markup", () => {
+    // A guard that found nothing to check would pass forever. If a
+    // refactor renames the directory, changes the helper, or moves the
+    // markup, this fails instead of quietly covering zero call sites.
+    // The floors are far below the counts measured when this was written
+    // (434 lookups across 20 of the 24 files under src/popup/, against 274
+    // ids in the markup), so ordinary churn does not trip them.
+    test("the scan actually found the code and the markup", () => {
+        const files = new Set(REFERENCES.map((r) => r.file));
+        expect(files.size).toBeGreaterThanOrEqual(15);
+        expect(REFERENCES.length).toBeGreaterThanOrEqual(300);
+        expect(HTML_IDS.length).toBeGreaterThanOrEqual(200);
+
+        // The densest screen, named explicitly: a scan that stopped
+        // covering src/popup/views/settings.js is the exact regression
+        // this file was written for.
+        expect(
+            files.has(path.join("src", "popup", "views", "settings.js")),
+        ).toBe(true);
+        expect(
+            REFERENCES.some((r) => r.id === "settings-hide-spoofed-symbols"),
+        ).toBe(true);
+        expect(REFERENCES.some((r) => r.id === "view-settings")).toBe(true);
+    });
+
+    test("no lookup names an id that src/popup/index.html does not define", () => {
+        const missing = REFERENCES.filter(
+            (r) => !HTML_ID_SET.has(r.id) && !RUNTIME_CREATED_IDS.has(r.id),
+        ).map(
+            (r) =>
+                r.file +
+                ":" +
+                r.line +
+                " " +
+                r.source +
+                ' looks up id "' +
+                r.id +
+                '", which is not in src/popup/index.html',
+        );
+
+        expect(missing).toEqual([]);
+    });
+
+    test("every id excused as runtime-created is still looked up somewhere", () => {
+        // Otherwise the exception list becomes a place stale names
+        // accumulate, and the next real miss can be waved through by
+        // adding one more.
+        for (const id of RUNTIME_CREATED_IDS) {
+            expect(REFERENCES.some((r) => r.id === id)).toBe(true);
+            expect(HTML_ID_SET.has(id)).toBe(false);
+        }
+    });
+
+    test("index.html defines no id twice", () => {
+        // getElementById returns the first match, so a duplicate id means
+        // one of the two elements can never be reached by the code that
+        // thinks it owns it.
+        const seen = new Set();
+        const duplicated = [];
+        for (const id of HTML_IDS) {
+            if (seen.has(id)) duplicated.push(id);
+            seen.add(id);
+        }
+
+        expect(duplicated).toEqual([]);
+    });
+});