fix(config): validate null hover background

src/config/config.test.ts

@@ -453,6 +453,30 @@ test('parses subtitleStyle.hoverBackground as a hoverTokenBackgroundColor alias'
   assert.equal(validService.getConfig().subtitleStyle.hoverTokenBackgroundColor, 'transparent');
 });
 
+test('parses subtitleStyle.hoverTokenBackgroundColor null as invalid instead of missing', () => {
+  const invalidDir = makeTempDir();
+  fs.writeFileSync(
+    path.join(invalidDir, 'config.jsonc'),
+    `{
+      "subtitleStyle": {
+        "hoverTokenBackgroundColor": null
+      }
+    }`,
+    'utf-8',
+  );
+
+  const invalidService = new ConfigService(invalidDir);
+  assert.equal(
+    invalidService.getConfig().subtitleStyle.hoverTokenBackgroundColor,
+    DEFAULT_CONFIG.subtitleStyle.hoverTokenBackgroundColor,
+  );
+  assert.ok(
+    invalidService
+      .getWarnings()
+      .some((warning) => warning.path === 'subtitleStyle.hoverTokenBackgroundColor'),
+  );
+});
+
 test('parses subtitleStyle.nameMatchEnabled and warns on invalid values', () => {
   const validDir = makeTempDir();
   fs.writeFileSync(

src/config/resolve/subtitle-domains.ts

@@ -265,7 +265,9 @@ export function applySubtitleDomainConfig(context: ResolveContext): void {
       hoverTokenBackgroundColor?: unknown;
     };
     const rawHoverTokenBackgroundColor =
-      subtitleStyleSource.hoverTokenBackgroundColor ?? subtitleStyleSource.hoverBackground;
+      subtitleStyleSource.hoverTokenBackgroundColor !== undefined
+        ? subtitleStyleSource.hoverTokenBackgroundColor
+        : subtitleStyleSource.hoverBackground;
     const hoverTokenBackgroundColor = asString(rawHoverTokenBackgroundColor);
     if (hoverTokenBackgroundColor !== undefined) {
       resolved.subtitleStyle.hoverTokenBackgroundColor = hoverTokenBackgroundColor;