=== Description
If I use the tab key to focus the User menu button, and then press the enter key, I would expect the menu to open but it does not. The same goes for the Language button (however when it is show in the site header, like on Commons or Office Wiki, it does open via tab + enter), the More button in the article toolbar, and any other Menu dropdown component.
{F34644119 width=600}
=== Developer notes
It might make sense to convert sidebar.js into a generic checkboxHack.js, which would handle enhancing all checkbox hacks in Vector. This would be a good opportunity for us to align with [[ https://github.com/wikimedia/mediawiki/blob/master/resources/src/mediawiki.page.ready/checkboxHack.js#L40-L60 | the checkbox hack markup defined in core ]] so that we are using common selectors (i.e. 'mw-checkbox-hack-checkbox', 'mw-checkbox-hack-button', and 'mw-checkbox-hack-target').
=== QA steps
Browser compatibility may be an issue here so please do the following steps with Edge, Safari, Firefox, and Chrome:
1) Visit https://en.wikipedia.beta.wmflabs.org/wiki/Dog
2) Tab until the hamburger menu button is in focus. Use the enter key to assert the sidebar can be opened/closed. Use the space key and assert the sidebar can be opened/closed. Use a mouse click and assert the sidebar can be opened/closed.
3) Tab until the user menu in the main header (not the sticky header) is in focus. Use the enter key to assert it can be opened/closed. Use the space key and assert it can be opened/closed. Use a mouse click and assert it can be opened/closed.
4) Tab until the "More" tab is in focus. Use the enter key to assert it can be opened/closed. Use the space key and assert it can be opened/closed. Use a mouse click and assert it can be opened/closed.