Fix window-restoration (sindresorhus#33)

* Suggested approach: sindresorhus/touch-bar-simulator:Touch%20Bar%20Simulator/TouchBarWindow.swift@23f7c1c#L121-L125
* use `visibleFrame` instead of `frame` to calculate the right coordinates
* Remove `window?.center()` and `showWindow()` which had no effect

Sources/Preferences/PreferencesWindowController.swift

@@ -1,5 +1,9 @@
 import Cocoa
 
+extension NSWindow.FrameAutosaveName {
+	static let preferences: NSWindow.FrameAutosaveName = "com.sindresorhus.Preferences.FrameAutosaveName"
+}
+
 public final class PreferencesWindowController: NSWindowController {
 	private let tabViewController = PreferencesTabViewController()
 
@@ -78,16 +82,28 @@ public final class PreferencesWindowController: NSWindowController {
 	/// - Parameter preferencePane: Identifier of the preference pane to display, or `nil` to show the
 	///   tab that was open when the user last closed the window.
 	public func show(preferencePane preferenceIdentifier: PreferencePane.Identifier? = nil) {
-		if !window!.isVisible {
-			window?.center()
-		}
-
-		showWindow(self)
 		if let preferenceIdentifier = preferenceIdentifier {
 			tabViewController.activateTab(preferenceIdentifier: preferenceIdentifier, animated: false)
 		} else {
 			tabViewController.restoreInitialTab()
 		}
+
+		showWindow(self)
+
+		restoreWindowPosition()
+
 		NSApp.activate(ignoringOtherApps: true)
 	}
+
+	private func restoreWindowPosition() {
+		guard let window = self.window,
+			let screenContainingWindow = window.screen 
+			else { return }
+
+		let x = screenContainingWindow.visibleFrame.midX - window.frame.width / 2
+		let y = screenContainingWindow.visibleFrame.midY - window.frame.height / 2
+		window.setFrameOrigin(CGPoint(x: x, y: y))
+		window.setFrameUsingName(.preferences)
+		window.setFrameAutosaveName(.preferences)
+	}
 }