A Kanban board opened in a split route tile rendered no board switcher:
the board contributed it to WORKSPACE_PAGE_HEADER_AREA unconditionally,
and only the workspace pane paints that area. The tile's contribution
also leaked into another page's header and shared its id with the full
page's, so closing the tile removed the page's switcher.
Add WorkspacePageHeaderControl (exported via the plugin SDK). The
workspace pane's render provides a private host context; inside it the
control projects into the page header, anywhere else it renders inline.
The board mounts BoardSwitcher once, through it, in its own header row.
Fixes#123597
Originally authored by Justin Haynes (@jhaynes).
aa25f9e85f