The SDK declares that completion handler @MainActor. Without the
annotation the method only nearly matched the optional
WKNavigationDelegate requirement — Swift's own wording in the warning —
so it was treated as an unrelated method that happened to resemble one,
and the delegate never called it.
The effect was not cosmetic: the .linkActivated branch never ran, so
tapping a link in an Org README navigated the embedded WKWebView to
that page rather than handing it to the browser. That happens inside a
frame sized to the README's original height, with scrolling disabled and
no back affordance.
One annotation:
decisionHandler: @escaping @MainActor (WKNavigationActionPolicy) -> Void
Build is warning-free afterwards and the unit suite passes. Worth a tap on a real external link to confirm the hand-off, which a test cannot cover.