build(frontend): fail css-check on a nested rule the phone drops

The device renders in Chrome 113, which predates relaxed CSS nesting, so
a nested rule whose selector starts with an element name is not a parse
error anyone would notice -- the rule simply does not exist, there and
nowhere else. Three were live in `index.css`, and the one that mattered
was the `text-overflow: ellipsis` on the bottom bar's title and artist,
which had therefore never truncated on the device. No tier here can see
the class at all: the component tier, the e2e tier and `make ui-visual`
all run a current engine, where the rule applies normally.

So `make css-check` carries a second script. It reads `index.css` and
the `css` literals in `src/**/*.ts` alike, since a shadow-root
stylesheet is parsed by the same engine, and it names the file, the line
and the fix -- a leading `&`, which is valid in both syntaxes.

The detection walks blocks rather than matching lines, and both things
it has to get right fall out of one rule: a rule is nested when a
*style* rule is somewhere above it, not when its immediate parent is a
block. That leaves `@media (...) { bottom-nav { ... } }` at the top
level alone, which is the majority of what a regex over the file would
report, and still flags the same rule inside an at-rule that is itself
inside a style rule. Strings and comments are read through, so a brace
in a `url()` is not a block.

The tree has no violation left, so the check would pass just as happily
over an empty glob: it refuses one, and `test/utils/css-nesting.test.ts`
pins the semantics that make the sweep mean something. The literal
scanner the two checks share is lifted into `css-literals.mjs`
unchanged, except that a `${}` substitution is now blanked keeping its
newlines so a line number survives it.

Closes #154

frontend/scripts/check-css-literals.mjs

@@ -23,97 +23,23 @@
  * as the literal contains an unterminated `/*`. Nothing else produces
  * that, and a legitimate literal cannot contain one.
  */
-import { readFileSync } from 'node:fs';
-import { globSync } from 'node:fs';
+import { globSync, readFileSync } from 'node:fs';
+
+import { taggedLiterals } from './css-literals.mjs';
 
 const TAGS = ['css', 'html', 'svg'];
 
-/**
- * Find the end of a template literal that starts at `start` (the index
- * of its opening backtick), respecting escapes and `${}` substitutions.
- * Returns the index of the closing backtick, or -1.
- */
-function endOfTemplate(src, start) {
-  let depth = 0;
-
-  for (let i = start + 1; i < src.length; i++) {
-    const c = src[i];
-
-    if (c === '\\') {
-      i++;
-      continue;
-    }
-
-    if (c === '$' && src[i + 1] === '{') {
-      depth++;
-      i++;
-      continue;
-    }
-
-    if (c === '}' && depth > 0) {
-      depth--;
-      continue;
-    }
-
-    if (c === '`' && depth === 0) return i;
-  }
-
-  return -1;
-}
-
-/** Strip `${...}` substitutions, which may legitimately contain anything. */
-function stripSubstitutions(text) {
-  let out = '';
-  let depth = 0;
-
-  for (let i = 0; i < text.length; i++) {
-    if (text[i] === '$' && text[i + 1] === '{') {
-      depth++;
-      i++;
-      continue;
-    }
-
-    if (text[i] === '}' && depth > 0) {
-      depth--;
-      continue;
-    }
-
-    if (depth === 0) out += text[i];
-  }
-
-  return out;
-}
-
-function lineOf(src, index) {
-  return src.slice(0, index).split('\n').length;
-}
-
 const files = globSync('src/**/*.ts', { cwd: process.cwd() });
 const problems = [];
 
 for (const file of files) {
   const src = readFileSync(file, 'utf8');
-  const tagPattern = new RegExp(`(^|[^\\w$.])(${TAGS.join('|')})\``, 'g');
 
-  let match;
-
-  while ((match = tagPattern.exec(src)) !== null) {
-    const open = match.index + match[0].length - 1;
-    const close = endOfTemplate(src, open);
-
-    if (close === -1) continue;
-
-    const body = stripSubstitutions(src.slice(open + 1, close));
+  for (const { tag, body, line } of taggedLiterals(src, TAGS)) {
     const opens = (body.match(/\/\*/g) ?? []).length;
     const closes = (body.match(/\*\//g) ?? []).length;
 
-    if (opens > closes) {
-      problems.push({
-        file,
-        line: lineOf(src, open),
-        tag: match[2],
-      });
-    }
+    if (opens > closes) problems.push({ file, line, tag });
   }
 }
 

frontend/scripts/check-css-nesting.mjs

@@ -0,0 +1,65 @@
+#!/usr/bin/env node
+/**
+ * Fail on a nested rule whose selector starts with an element name.
+ *
+ * See `css-nesting.mjs` for what the phone does with one. No tier here
+ * can see it: the component tier, the e2e tier and `make ui-visual` all
+ * run a current Chromium, where the rule applies normally, so the only
+ * report is a screenshot of the device — which is how the bottom bar's
+ * title came to have never truncated there.
+ *
+ * It covers `index.css` and the `css` literals in the components alike,
+ * because a shadow-root stylesheet is parsed by the same engine.
+ */
+import { globSync, readFileSync } from 'node:fs';
+
+import { taggedLiterals } from './css-literals.mjs';
+import { findBareNestedRules } from './css-nesting.mjs';
+
+const problems = [];
+
+for (const { line, selector } of findBareNestedRules(
+  readFileSync('index.css', 'utf8'),
+)) {
+  problems.push({ file: 'index.css', line, selector });
+}
+
+const sources = globSync('src/**/*.ts', { cwd: process.cwd() });
+
+// A sweep over an empty glob passes, and this one is expected to find
+// nothing, so "it found nothing" has to mean it looked.
+if (sources.length === 0) {
+  console.error('css-nesting-check: no sources matched src/**/*.ts');
+  process.exit(1);
+}
+
+for (const file of sources) {
+  const src = readFileSync(file, 'utf8');
+
+  for (const literal of taggedLiterals(src, ['css'])) {
+    for (const { line, selector } of findBareNestedRules(literal.body)) {
+      problems.push({ file, line: literal.line + line - 1, selector });
+    }
+  }
+}
+
+if (problems.length > 0) {
+  for (const p of problems) {
+    console.error(
+      `${p.file}:${p.line}: nested rule "${p.selector.split('\n')[0]}" starts ` +
+        'with an element name — write it as "& ' +
+        `${p.selector.split('\n')[0]}"`,
+    );
+  }
+
+  console.error(
+    `\ncss-nesting-check: ${problems.length} problem(s). ` +
+      'Chrome 113 (the device) drops a nested rule that does not start ' +
+      'with a symbol; the leading & is valid in both syntaxes.',
+  );
+  process.exit(1);
+}
+
+console.log(
+  `css-nesting-check: index.css + ${sources.length} files, no bare nested rules`,
+);

frontend/scripts/css-literals.mjs

@@ -0,0 +1,103 @@
+/**
+ * Finding the `css` tagged templates in a TypeScript source.
+ *
+ * Two checks read them — the unterminated-comment one and the nesting
+ * one — and a second scanner would be a second thing to keep in step
+ * with how a template literal actually ends.
+ */
+
+/**
+ * Find the end of a template literal that starts at `start` (the index
+ * of its opening backtick), respecting escapes and `${}` substitutions.
+ * Returns the index of the closing backtick, or -1.
+ */
+export function endOfTemplate(src, start) {
+  let depth = 0;
+
+  for (let i = start + 1; i < src.length; i++) {
+    const c = src[i];
+
+    if (c === '\\') {
+      i++;
+      continue;
+    }
+
+    if (c === '$' && src[i + 1] === '{') {
+      depth++;
+      i++;
+      continue;
+    }
+
+    if (c === '}' && depth > 0) {
+      depth--;
+      continue;
+    }
+
+    if (c === '`' && depth === 0) return i;
+  }
+
+  return -1;
+}
+
+/**
+ * Strip `${...}` substitutions, which may legitimately contain anything.
+ *
+ * Newlines inside them are kept, so a line number taken from the
+ * stripped text still names the right line of the file it came from.
+ */
+export function stripSubstitutions(text) {
+  let out = '';
+  let depth = 0;
+
+  for (let i = 0; i < text.length; i++) {
+    if (text[i] === '$' && text[i + 1] === '{') {
+      depth++;
+      i++;
+      continue;
+    }
+
+    if (text[i] === '}' && depth > 0) {
+      depth--;
+      continue;
+    }
+
+    if (depth === 0) out += text[i];
+    else if (text[i] === '\n') out += '\n';
+  }
+
+  return out;
+}
+
+/** The 1-based line number of `index` in `src`. */
+export function lineOf(src, index) {
+  return src.slice(0, index).split('\n').length;
+}
+
+/**
+ * Every tagged template literal in `src` whose tag is in `tags`.
+ *
+ * `body` has its substitutions stripped and `line` is the line its
+ * opening backtick sits on, so `line + (n - 1)` is the file line of the
+ * body's own line `n`.
+ */
+export function taggedLiterals(src, tags) {
+  const pattern = new RegExp(`(^|[^\\w$.])(${tags.join('|')})\``, 'g');
+  const found = [];
+
+  let match;
+
+  while ((match = pattern.exec(src)) !== null) {
+    const open = match.index + match[0].length - 1;
+    const close = endOfTemplate(src, open);
+
+    if (close === -1) continue;
+
+    found.push({
+      tag: match[2],
+      body: stripSubstitutions(src.slice(open + 1, close)),
+      line: lineOf(src, open),
+    });
+  }
+
+  return found;
+}

frontend/scripts/css-nesting.mjs

@@ -0,0 +1,119 @@
+/**
+ * A nested rule whose selector starts with an element name is silently
+ * dropped on the phone.
+ *
+ * The device renders in Chrome 113, which predates relaxed CSS nesting
+ * (Chrome 120): before that a nested selector had to start with
+ * something that could not be read as the beginning of a declaration,
+ * so `.bottom-bar { audio-player { … } }` is not a parse error anyone
+ * would notice — the inner rule simply does not exist, on the phone and
+ * only on the phone. Three were live in `index.css`, one of them the
+ * `text-overflow: ellipsis` on the bottom bar's title, which had
+ * therefore never truncated on the device.
+ *
+ * `& audio-player` is valid in both syntaxes, so no nested rule here
+ * has any reason to omit it.
+ *
+ * Two things the detection has to get right:
+ *
+ * - **A rule directly inside an at-rule is not nested.**
+ *   `@media (…) { bottom-nav { … } }` at the top level is an ordinary
+ *   rule and is fine — and it is the majority of the matches a regex
+ *   over the file would produce. What decides it is whether a *style*
+ *   rule is somewhere above, not what the immediate parent is: inside
+ *   `.bar { @media (…) { audio-player { … } } }` the inner rule is
+ *   nested, at-rule in between or not.
+ * - **A declaration is not a rule.** `background: url(…)` and any
+ *   string or comment can hold a brace, so this tracks them rather than
+ *   matching lines.
+ */
+
+/** Does this selector start with an identifier, rather than a symbol? */
+function startsWithIdent(selector) {
+  return /^[A-Za-z_\u00A0-\uFFFF]/.test(selector);
+}
+
+/**
+ * Every nested style rule in `css` whose selector starts with an
+ * element name, as `{ line, selector }` with a 1-based line.
+ */
+export function findBareNestedRules(css) {
+  const found = [];
+  /** The blocks we are inside, innermost last: 'style' or 'at'. */
+  const stack = [];
+  /** The text since the last `{`, `}` or `;` — a prelude, if a `{` follows. */
+  let prelude = '';
+  let preludeLine = 1;
+  let line = 1;
+
+  const startPrelude = () => {
+    prelude = '';
+    preludeLine = line;
+  };
+
+  for (let i = 0; i < css.length; i++) {
+    const c = css[i];
+
+    if (c === '\n') {
+      line++;
+      if (prelude.trim() === '') preludeLine = line;
+      prelude += c;
+      continue;
+    }
+
+    if (c === '/' && css[i + 1] === '*') {
+      const end = css.indexOf('*/', i + 2);
+      const comment = css.slice(i, end === -1 ? css.length : end + 2);
+
+      line += (comment.match(/\n/g) ?? []).length;
+      i += comment.length - 1;
+      if (prelude.trim() === '') preludeLine = line;
+      continue;
+    }
+
+    if (c === '"' || c === "'") {
+      let j = i + 1;
+
+      while (j < css.length && css[j] !== c) {
+        if (css[j] === '\\') j++;
+        j++;
+      }
+
+      prelude += css.slice(i, j + 1);
+      i = j;
+      continue;
+    }
+
+    if (c === '{') {
+      const selector = prelude.trim();
+      const kind = selector.startsWith('@') ? 'at' : 'style';
+
+      if (
+        kind === 'style' &&
+        stack.includes('style') &&
+        startsWithIdent(selector)
+      ) {
+        found.push({ line: preludeLine, selector });
+      }
+
+      stack.push(kind);
+      startPrelude();
+      continue;
+    }
+
+    if (c === '}') {
+      stack.pop();
+      startPrelude();
+      continue;
+    }
+
+    if (c === ';') {
+      startPrelude();
+      continue;
+    }
+
+    prelude += c;
+  }
+
+  return found;
+}