Present asset picker as popover in regular horizontal size class

Tusker/Screens/Asset Picker/AssetCollectionViewController.swift

@@ -49,6 +49,21 @@ class AssetCollectionViewController: UICollectionViewController {
     override func viewDidLoad() {
         super.viewDidLoad()
         
+        // use the safe area layout guide instead of letting it automatically use the safe area insets
+        // because otherwise, when presented in a popover with the arrow on the left or right side,
+        // the collection view content will be cut off by the width of the arrow because the popover
+        // doesn't respect safe area insets
+        collectionView.translatesAutoresizingMaskIntoConstraints = false
+        NSLayoutConstraint.activate([
+            view.safeAreaLayoutGuide.leadingAnchor.constraint(equalTo: collectionView.leadingAnchor),
+            view.safeAreaLayoutGuide.trailingAnchor.constraint(equalTo: collectionView.trailingAnchor),
+            // top ignores safe area because when presented in the sheet container, it simplifies the top content offset
+            view.topAnchor.constraint(equalTo: collectionView.topAnchor),
+            // bottom ignores safe area because we want cells to underflow bottom of the screen on notched iPhones
+            view.bottomAnchor.constraint(equalTo: collectionView.bottomAnchor),
+        ])
+        view.backgroundColor = .systemBackground
+        
         navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .done, target: self, action: #selector(donePressed))
 
         collectionView.alwaysBounceVertical = true

Tusker/Screens/Compose/ComposeAttachmentsViewController.swift

@@ -303,10 +303,20 @@ class ComposeAttachmentsViewController: UITableViewController {
     // MARK: Interaction
     
     func addAttachmentPressed() {
+        if traitCollection.horizontalSizeClass == .compact {
             let sheetContainer = AssetPickerSheetContainerViewController()
             sheetContainer.assetPicker.assetPickerDelegate = self
             present(sheetContainer, animated: true)
-        //setOverrideTraitCollection(UITraitCollection(userInterfaceLevel: .elevated), forChild: sheetContainer)
+        } else {
+            let picker = AssetPickerViewController()
+            picker.assetPickerDelegate = self
+            picker.overrideUserInterfaceStyle = .dark
+            picker.modalPresentationStyle = .popover
+            present(picker, animated: true)
+            if let presentationController = picker.presentationController as? UIPopoverPresentationController {
+                presentationController.sourceView = tableView.cellForRow(at: IndexPath(row: 0, section: 1))
+            }
+        }
     }
 
 }