forkBranch was unconditionally routing every branched session into the
main pane via resumeSession, including sidebar/background branches of
a session the user isn't currently viewing. That reintroduces the
#69750 focus-stealing bug for that path: branching a different session
from the sidebar yanked the active view away from whatever was open.
Only take over the main pane when the branch's parent is the session
already selected; otherwise keep opening it as its own tile.
chelsealong
9fbe3cc040