refresh the bars on screen changes

Tests/phbarTests/AppDelegateTests.swift

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+import AppKit
+import Foundation
+import Testing
+
+@testable import phbar
+
+@MainActor
+struct AppDelegateTests {
+
+	// MARK: - ScreenSync.diff
+
+	@Test func diffReportsAddedWhenNothingOccupied() throws {
+		let screen = try #require(NSScreen.main)
+		let plan = PHScreenSync.diff(current: [screen], occupied: [])
+		#expect(plan.added == [screen])
+		#expect(plan.removed.isEmpty)
+		#expect(plan.kept.isEmpty)
+	}
+
+	@Test func diffReportsRemovedWhenScreenGone() throws {
+		let screen = try #require(NSScreen.main)
+		let plan = PHScreenSync.diff(current: [], occupied: [screen])
+		#expect(plan.removed == [screen])
+		#expect(plan.added.isEmpty)
+		#expect(plan.kept.isEmpty)
+	}
+
+	@Test func diffKeepsScreenStillAttached() throws {
+		let screen = try #require(NSScreen.main)
+		let plan = PHScreenSync.diff(current: [screen], occupied: [screen])
+		#expect(plan.kept == [screen])
+		#expect(plan.added.isEmpty)
+		#expect(plan.removed.isEmpty)
+	}
+
+	/// A disconnected monitor reappears as a *new* `NSScreen` instance, so the
+	/// same physical display must be reported as remove + add, never a keep.
+	@Test func diffTreatsDistinctInstancesAsRemoveThenAdd() throws {
+		let main = try #require(NSScreen.main)
+		guard let other = NSScreen.screens.first(where: { $0 !== main }) else {
+			// Single-monitor environment: nothing to contrast against. Verify
+			// identity semantics hold for the same object re-presented instead.
+			let kept = PHScreenSync.diff(current: [main], occupied: [main])
+			#expect(kept.kept == [main])
+			return
+		}
+
+		let plan = PHScreenSync.diff(current: [other], occupied: [main])
+		#expect(plan.removed == [main])
+		#expect(plan.added == [other])
+		#expect(plan.kept.isEmpty)
+	}
+
+	@Test func diffIgnoresOrdering() throws {
+		let main = try #require(NSScreen.main)
+		guard let other = NSScreen.screens.first(where: { $0 !== main }) else {
+			// Single-monitor environment: ordering is trivially stable.
+			return
+		}
+		let plan = PHScreenSync.diff(current: [other, main], occupied: [main, other])
+		#expect(Set(plan.kept.map(ObjectIdentifier.init)) == Set([main, other].map(ObjectIdentifier.init)))
+		#expect(plan.added.isEmpty)
+		#expect(plan.removed.isEmpty)
+	}
+
+	// MARK: - BarControllerFactory
+
+	@Test func factoryBuildsControllerForScreen() throws {
+		let screen = try #require(NSScreen.main)
+		let config = try PHConfig.load()
+		let factory = PHBarFactory(config: config, environment: .process, debug: false)
+
+		let controller = try factory.make(for: screen)
+
+		#expect(controller.screen === screen)
+		#expect(controller.blocks.isEmpty == false)
+	}
+}