When a menupopup is opened, the active element does not change, so
their focus-ring will be hidden until the menupopup closes for it to be
less distracting.
When a panel popup is opened, the focus will be moved inside
of the panel, so we keep track of whichever element was previously
focused, and re-focus it when the panel goes away.
Minor reorganization of focus-ring mixin to use variables instead
of parameters to make hiding of focus-ring easier.
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