webcomponent: transfer focusable elements during re-renders

[PatrickJS]

certain elements may be the current activeElement. The list (from https://github.com/WICG/inert/blob/09c0647f59a3ad61ffbd92d17b9a0770fdbef412/src/inert.js#L24)

• a[href]
• area[href]
• input:not([disabled])
• select:not([disabled])
• textarea:not([disabled])
• button:not([disabled])
• details
• summary
• iframe
• object
• embed
• [contenteditable]