perf(theme): scope prefixes by last dot in TokenStyleResolver (#1530)

lib/core/theme/parser/token_style_resolver.dart

@@ -22,21 +22,19 @@ class TokenStyleResolver {
       _cache.putIfAbsent(scope, () => _resolveUncached(scope));
 
   TextStyle _resolveUncached(String scope) {
-    for (final prefix in _scopePrefixes(scope)) {
+    // `a.b.c` -> `a.b` -> `a`: cut at the last dot instead of splitting into a
+    // list and joining sublists (#1530).
+    var prefix = scope;
+    while (true) {
       for (final rule in _rules) {
         if (rule.scopes.contains(prefix)) {
           return _styleFromRule(rule);
         }
       }
+      final dot = prefix.lastIndexOf('.');
+      if (dot < 0) return _defaultStyle;
+      prefix = prefix.substring(0, dot);
     }
-    return _defaultStyle;
-  }
-
-  List<String> _scopePrefixes(String scope) {
-    final parts = scope.split('.');
-    return [
-      for (var i = parts.length; i >= 1; i--) parts.sublist(0, i).join('.'),
-    ];
   }
 
   TextStyle _styleFromRule(TokenColorRule rule) {

test/core/theme/parser/token_style_resolver_test.dart

@@ -37,4 +37,30 @@ void main() {
     final b = resolver.resolve('string.quoted.double');
     expect(identical(a, b), isTrue);
   });
+
+  test('walks prefixes by dot, including empty segments (#1530)', () {
+    const rules = [
+      TokenColorRule(scopes: ['a'], foreground: '#010101'),
+      TokenColorRule(scopes: ['a.b'], foreground: '#020202'),
+      TokenColorRule(scopes: ['a.'], foreground: '#030303'),
+      TokenColorRule(scopes: [''], foreground: '#040404'),
+      TokenColorRule(scopes: ['x.y.z'], foreground: '#050505'),
+    ];
+    final resolver = TokenStyleResolver(
+      rules: rules,
+      defaultStyle: const TextStyle(color: Color(0xFFFFFFFF)),
+    );
+
+    // The longest prefix wins.
+    expect(resolver.resolve('a.b.c.d').color, const Color(0xFF020202));
+    expect(resolver.resolve('a.c.d').color, const Color(0xFF010101));
+    expect(resolver.resolve('x.y.z.w').color, const Color(0xFF050505));
+    // `a..b` -> `a.` -> `a`: the empty segment is a prefix of its own.
+    expect(resolver.resolve('a..b').color, const Color(0xFF030303));
+    // The leading dot gives the empty prefix; a bare empty scope is it too.
+    expect(resolver.resolve('.q').color, const Color(0xFF040404));
+    expect(resolver.resolve('').color, const Color(0xFF040404));
+    // Nothing matches: the default style.
+    expect(resolver.resolve('zzz.top').color, const Color(0xFFFFFFFF));
+  });
 }