[gtritchie]

Fork-side fixes for RStudio Server's main menubar, which deviates from the WAI-ARIA APG menubar pattern in several places. Addresses rstudio/rstudio#19324.

These changes are in MenuBar and MenuItem, so they apply to every GWT menu in RStudio, not just the main menu: toolbar menus, context menus, and the source tab overflow list too.

Changes

Keyboard

• Down, Right, Enter and Space focus the submenu's first item. Down on a menubar item and Right on a submenu parent did nothing once the mouse had selected an item in the open submenu, because hovering selects without moving focus. Enter and Space opened a submenu but left focus on its parent. All four now open the submenu if needed and focus its first item. An item's command still takes precedence over its submenu for Enter and Space. Enter and Space on a menubar item whose submenu is already open, e.g. after a click, now move into it rather than closing it, as APG specifies; clicking the item still closes it.
• Right on a leaf two levels down moves to the next menubar item instead of back to the start of its own submenu, and Down on a horizontal leaf likewise walks ancestors without re-entering submenus.
• Up moves within a menu whose submenu was opened by hovering. It used to close the whole menu, or do nothing in a root menu.
• Disabled items can be reached with the arrow keys, Home and End, as APG specifies; they still can't be activated. Moving onto a disabled item, a menubar item with no submenu, or one whose submenu is empty, closes the submenu that was open rather than leaving it open under the new selection. Empty submenus aren't opened from the keyboard at all.
• Tab and Shift+Tab move relative to the root menu, not the popup appended to the end of <body>. Tab focuses the root element directly when focus is in a submenu, then closes the menus with a flag set on the root that stops onPopupClosed() from refocusing any of them. Closing a submenu otherwise focuses its parent, and MenuBar.focus() is deferred on the safari user agent (Chrome, Edge and Safari), so that focus would run after the browser's default Tab action and pull focus back into the menubar. Other paths that close submenus still refocus, as before.

ARIA

• An open submenu is owned by its parent menu and named by its parent item. While open, the parent menu sets aria-owns to it, so assistive technology shows it inside the menubar instead of as a separate top-level node, and the submenu is aria-labelledby the item that opened it, unless the app has already given it aria-label or aria-labelledby. Both attributes are removed when the submenu closes. aria-owns on the container follows APG's guidance for menus that aren't DOM siblings of their parent item.
• aria-expanded is false on collapsed submenu parents rather than removed. It is still removed when an item's submenu is cleared.

Testing

New and updated tests in MenuBarTest and MenuItemTest cover each change. The keyboard tests drive the menus with dispatched key events, the focus checks wait for deferred focus, and the disabled-item test checks commands after scheduled commands have run.

• MenuBarTest and MenuItemTest pass in HtmlUnit except testSetFocusOnHoverDisabled, which fails the same way on rstudio/parrot-balsam without these changes.
• The tests can't catch the deferred-focus problems the Tab changes fix. HtmlUnit's default user agent focuses synchronously, FocusPanel.impl is static final so a test can't substitute the deferred implementation, and under -runStyle HtmlUnit:Chrome focus assertions fail even in the existing testSetFocusOnHoverEnabled. Those paths were checked in Chrome instead: Tab and Shift+Tab with one menu open by click, two levels open by click and hover, and from inside a submenu.
• checkstyle passes, and ant clean dist-dev builds.
• With RStudio Server built against this SDK, in Chrome on macOS: each behavior above was checked in the main menu, including Tab from an open menu landing where Tab from the closed menubar does. Axe reports no violations with a menu open inside the menubar.

The RStudio-side changes (focus restoration, the menubar's name, shortcut entry) are in a separate RStudio PR, which will also move GWT_VERSION to the release containing this.