Only restyle viewport-relative nodes on viewport size change

tests/unit/style/value.rs

@@ -0,0 +1,15 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use app_units::Au;
+use style::values::HasViewportPercentage;
+use style::values::specified::{ViewportPercentageLength, Length};
+
+#[test]
+fn length_has_viewport_percentage() {
+    let l = Length::ViewportPercentage(ViewportPercentageLength::Vw(100.));
+    assert!(l.has_viewport_percentage());
+    let l = Length::Absolute(Au(100));
+    assert!(!l.has_viewport_percentage());
+}