style: limit emoji borders to selected and hovered tiles

Sources/Conjure/EmojiPickerController.swift

@@ -284,7 +284,7 @@ final class EmojiPickerController: NSViewController, NSCollectionViewDataSource,
 
 @MainActor
 private final class EmojiItem: NSCollectionViewItem {
-    let button = NSButton()
+    let button = EmojiButton()
     var clicked: (() -> Void)?
 
     override func loadView() {
@@ -294,7 +294,6 @@ private final class EmojiItem: NSCollectionViewItem {
         button.action = #selector(copyEmoji)
         button.wantsLayer = true
         button.layer?.cornerRadius = 10
-        button.layer?.borderWidth = 0.5
         button.layer?.borderColor = SunsetPalette.secondaryText.withAlphaComponent(0.22).cgColor
         view = button
     }
@@ -302,7 +301,33 @@ private final class EmojiItem: NSCollectionViewItem {
     func highlight(_ selected: Bool) {
         button.layer?.backgroundColor = selected ? SunsetPalette.accent.withAlphaComponent(0.22).cgColor : nil
         button.setAccessibilitySelected(selected)
+        button.selected = selected
     }
 
     @objc private func copyEmoji() { clicked?() }
 }
+
+@MainActor
+private final class EmojiButton: NSButton {
+    var selected = false { didSet { updateBorder() } }
+    private var hovered = false { didSet { updateBorder() } }
+    private var hoverTracking: NSTrackingArea?
+
+    override func updateTrackingAreas() {
+        super.updateTrackingAreas()
+        if let hoverTracking { removeTrackingArea(hoverTracking) }
+        let tracking = NSTrackingArea(rect: .zero,
+                                      options: [.mouseEnteredAndExited, .activeInKeyWindow, .inVisibleRect],
+                                      owner: self, userInfo: nil)
+        addTrackingArea(tracking)
+        hoverTracking = tracking
+        hovered = window.map { visibleRect.contains(convert($0.mouseLocationOutsideOfEventStream, from: nil)) } ?? false
+    }
+
+    override func mouseEntered(with event: NSEvent) { hovered = true }
+    override func mouseExited(with event: NSEvent) { hovered = false }
+
+    private func updateBorder() {
+        layer?.borderWidth = selected || hovered ? 0.5 : 0
+    }
+}