Element.attachShadow() throws NotSupportedError unconditionally for a
customized built-in (<button is="hive-btn">): the spec only allows
autonomous custom elements or a fixed list of native tags to host a
shadow root, and explicitly excludes any is=-upgraded built-in
regardless of which tag it upgrades. button isn't on that list either
way. This made every themed dialog (any confirm/prompt, since openDialog
always renders at least one button) throw and fail to render in a real
browser, though it passed CI since nothing there exercises actual
browser DOM.
hive-dialog and hive-toast are unaffected — both are genuine autonomous
custom elements (extends HTMLElement, no is= upgrade), which are valid
shadow hosts.
Fix: hive-btn no longer calls attachShadow. Styles adopt onto document
once (module-level guard) instead of per-instance shadow root, scoped
via the [is="hive-btn"] attribute selector instead of :host — same
light-DOM approach the rest of the app's .btn consumers already use.
Native button behaviour is untouched, only the styling mechanism
changed. Build clean.
b201f6be88