rustdoc: put focus on the help link when opening it from keyboard

This prevents some strange blur-event-related bugs with the "?" command
by ensuring that the focus remains in the same spot when the settings
area closes.

Author: notriddle

src/librustdoc/html/static/js/main.js

@@ -1082,6 +1082,9 @@ function loadCss(cssUrl) {
      * Show the help popup menu.
      */
     function showHelp() {
+        // Prevent `blur` events from being dispatched as a result of closing
+        // other modals.
+        getHelpButton().querySelector("a").focus();
         const menu = getHelpMenu(true);
         if (menu.style.display === "none") {
             window.hideAllModals();

tests/rustdoc-gui/settings.goml

@@ -203,6 +203,15 @@ press-key: "?"
 wait-for-css: ("#help-button .popover", {"display": "block"})
 assert-css: ("#settings-menu .popover", {"display": "none"})
 
+// Now switch back to the settings popover, and make sure the keyboard
+// shortcut works when a check box is selected.
+click: "#settings-menu > a"
+wait-for-css: ("#settings-menu .popover", {"display": "block"})
+focus: "#auto-hide-large-items"
+press-key: "?"
+wait-for-css: ("#settings-menu .popover", {"display": "none"})
+wait-for-css: ("#help-button .popover", {"display": "block"})
+
 // Now we go to the settings page to check that the CSS is loaded as expected.
 goto: "file://" + |DOC_PATH| + "/settings.html"
 wait-for: "#settings"