ReadonlycontainerThe Element to render into. Already styled to fill the dock's
content area; companions can appendChild and inline-style as
they would any container.
ReadonlyidReadonlylabelMake this the visible tab.
Simulate the user clicking × on the tab — fires onClose (so the
companion can tear down its listeners), then detaches.
Remove the tab + container without invoking onClose. Use this
from the companion's own unmount() to avoid recursion.
A panel registered with the dock. The companion gets back this handle and renders its UI into
container.