Also add a global `browserIsSelected` context property for all triggers,
so you can entirely avoid showing a message if the triggering browser
was in the background. You can use the selector and the context property
together or separately.
Context targeting:
`browserIsSelected`
Background tab selector:
`#tabbrowser-tabs:not([overflow]):not([haspinnedtabs]) %triggerTab%`
Current tab selector:
`#tabbrowser-tabs:not([overflow]):not([haspinnedtabs]) %triggerTab%[visuallyselected]`
Child of tab selector:
`#tabbrowser-tabs:not([overflow]):not([haspinnedtabs]) %triggerTab% .tab-icon-image`
Differential Revision: https://phabricator.services.mozilla.com/D212494
039ba4cf7a
