refresh the bars on screen changes

Sources/phbar/Models/PHScreenSync.swift

@@ -0,0 +1,31 @@
+import AppKit
+
+/// Which screens to keep, remove, or add when reconciling live bars against the
+/// current `NSScreen.screens`.
+///
+/// Pure and identity-based (it never reads `NSScreen` properties), so it can be
+/// unit-tested with any `[NSScreen]` without driving real window lifecycle.
+enum PHScreenSync {
+	struct Diff: Equatable {
+		/// Screens that already have a bar and are still attached. Their window
+		/// frame is recomputed since geometry may have changed.
+		let kept: [NSScreen]
+		/// Screens whose bar must be torn down (monitor disconnected).
+		let removed: [NSScreen]
+		/// Screens with no bar yet that need one (monitor connected).
+		let added: [NSScreen]
+	}
+
+	/// Compare the screens now attached (`current`) against the screens currently
+	/// occupied by bars (`occupied`). A disconnected monitor reappears as a new
+	/// `NSScreen` instance, so it is reported as remove + add, never a keep.
+	static func diff(current: [NSScreen], occupied: [NSScreen]) -> Diff {
+		let currentIDs = Set(current.map(ObjectIdentifier.init))
+		let occupiedIDs = Set(occupied.map(ObjectIdentifier.init))
+		return Diff(
+			kept: occupied.filter { currentIDs.contains(ObjectIdentifier($0)) },
+			removed: occupied.filter { !currentIDs.contains(ObjectIdentifier($0)) },
+			added: current.filter { !occupiedIDs.contains(ObjectIdentifier($0)) }
+		)
+	}
+}