Tabs opened from links on AppKit can be created before their URL is
loaded. That left background open-url tabs with the location field as
their stored responder, so switching to them later restored address-bar
focus instead of web content focus.
Track each tab's preferred responder, restore it when AppKit makes
the tab key, and mark page-backed open-url tabs to prefer the web view.
Blank new-tab pages still keep the location field focused.
