Fix visibility of mono-line <select>

.changeset/famous-badgers-drive.md

@@ -0,0 +1,5 @@
+---
+"@siteimprove/alfa-style": patch
+---
+
+**Fixed:** Mono-line `<select>` are again correctly considered as visible.

packages/alfa-style/src/node/predicate/is-visible.ts

@@ -102,8 +102,14 @@ function isInvisible(
 /**
  * Elements that are *not* "replaced elements" but are nonetheless visible when
  * empty
+ *
+ * @remarks
+ * * `<textarea>` always draw a box to input the text and thus are always visible
+ *   even though they have no children.
+ * * Single line `<select>` do not show their children `<option>` but nonetheless
+ *   show the text of the selected (or first) option, and thus are visible.
  */
-const isVisibleWhenEmpty = hasName("textarea");
+const isVisibleWhenEmpty = hasName("textarea", "select");
 
 /**
  * Does the element have set dimensions?

packages/alfa-style/test/node/predicate/is-visible.spec.tsx

@@ -349,3 +349,17 @@ test("isVisible() returns false for `<option>` that are not shown, and their con
     }
   }
 });
+
+test("isVisible() returns true for single line `<select>`", (t) => {
+  const options = [<option>one</option>, <option>two</option>];
+
+  const select = <select>{options}</select>;
+
+  for (const node of options) {
+    // None of the `<option>` is visible, due to being a single line `<select>`
+    t(!isVisible(node));
+  }
+
+  // The `<select>` is nonetheless visible, showing the value of the first `<option>`.
+  t(isVisible(select));
+});