rustdoc: add test and bug fix for theme defaults

src/librustdoc/html/static/js/storage.js

@@ -158,6 +158,7 @@ const updateTheme = (function() {
         if (getSettingValue("use-system-theme") !== "false") {
             const lightTheme = getSettingValue("preferred-light-theme") || "light";
             const darkTheme = getSettingValue("preferred-dark-theme") || "dark";
+            updateLocalStorage("use-system-theme", "true");
 
             if (mql.matches) {
                 use(darkTheme, true);

tests/rustdoc-gui/theme-defaults.goml

@@ -0,0 +1,24 @@
+// Ensure that the theme picker always starts with the actual defaults.
+goto: "file://" + |DOC_PATH| + "/test_docs/index.html"
+click: "#settings-menu"
+wait-for: "#theme-system-preference"
+assert: "#theme-system-preference:checked"
+assert: "#preferred-light-theme-light:checked"
+assert: "#preferred-dark-theme-dark:checked"
+assert-false: "#preferred-dark-theme-ayu:checked"
+
+// Test legacy migration from old theme setup without system-preference matching.
+// See https://github.com/rust-lang/rust/pull/77809#issuecomment-707875732
+local-storage: {
+    "rustdoc-preferred-light-theme": null,
+    "rustdoc-preferred-dark-theme": null,
+    "rustdoc-use-system-theme": null,
+    "rustdoc-theme": "ayu"
+}
+goto: "file://" + |DOC_PATH| + "/test_docs/index.html"
+click: "#settings-menu"
+wait-for: "#theme-system-preference"
+assert: "#theme-system-preference:checked"
+assert: "#preferred-light-theme-light:checked"
+assert-false: "#preferred-dark-theme-dark:checked"
+assert: "#preferred-dark-theme-ayu:checked"