[menu explainer] Update list of discussion issues. (#1444)

Author: dbaron

site/src/pages/components/menu.explainer.mdx

@@ -819,3 +819,26 @@ This section links to all of the relevant discussions and [issues](https://githu
 - [Define a default checked attribute for menuitem](https://github.com/openui/open-ui/issues/1205)
 - [Keyboard shortcuts?](https://github.com/openui/open-ui/issues/1225)
 - [Should menulist be a popover by default?](https://github.com/openui/open-ui/issues/1226)
+- [[menu] Soft disabling menuitem elements](https://github.com/openui/open-ui/issues/1274)
+- [Is aria-owns the right relationship for sub-menus?](https://github.com/openui/open-ui/issues/1297)
+- [[menu] Should an event be fired when a `<menuitem>` is selected? Which event?](https://github.com/openui/open-ui/issues/1312)
+- [What event should fire on checkable menu items?](https://github.com/openui/open-ui/issues/1321)
+- [How to support interactive content like `<input type=search>` in a menu?](https://github.com/openui/open-ui/issues/1323)
+- [Can we give menuitems a default command?](https://github.com/openui/open-ui/issues/1326)
+- [[focusgroup] Feedback: Mousepointer hover on other items should re-focus](https://github.com/openui/open-ui/issues/1406)
+- [[focusgroup] Feedback: Pointer-activated menu should not focus first item, but clicking down arrow after clicking the menu trigger should still activate the first item ( autofocus or tab-into should not be necessary)](https://github.com/openui/open-ui/issues/1407)
+- [[focusgroup] Moving focus between horizontal submenus](https://github.com/openui/open-ui/issues/1408)
+- [menubar/menulist/menuitem content model](https://github.com/openui/open-ui/issues/1433)
+- [What should the tab key do in menu elements?](https://github.com/openui/open-ui/issues/1438)
+- [[menu] moving keyboard focus into submenus](https://github.com/openui/open-ui/issues/1439)
+- [Do we need command=show-menu and command=hide-menu?](https://github.com/openui/open-ui/issues/1442)
+
+### WHATWG
+- [Introduce the new menu elements](https://github.com/whatwg/html/pull/12011)
+  - This is the HTML spec PR to add the new elements
+
+### CSSWG / WHATWG / OpenUI Task Force
+
+- [[menu] support :open pseudo-class on `<menuitem>`s that open submenus](https://github.com/openui/open-ui/issues/1363)
+- [[css-ui] Add or reuse pseudo-element for openable-submenu indicator (triangle)](https://github.com/w3c/csswg-drafts/issues/13430)
+  - This issue has the discussion that chose the name of the `::expand-icon` pseudo-element.