Fix compose toolbar being hidden by software keyboard on iPadOS 15

Closes #252

Tusker/Screens/Compose/ComposeView.swift

@@ -49,6 +49,7 @@ struct ComposeView: View {
     @State private var globalFrameOutsideList: CGRect = .zero
     @State private var contentWarningBecomeFirstResponder = false
     @State private var mainComposeTextViewBecomeFirstResponder = false
+    @StateObject private var keyboardReader = KeyboardReader()
     
     @OptionalStateObject private var poster: PostService?
     @State private var isShowingPostErrorAlert = false
@@ -107,6 +108,8 @@ struct ComposeView: View {
                     
                     ComposeToolbar(draft: draft)
                 }
+                // on iPadOS15, the toolbar ends up below the keyboard's toolbar without this
+                .padding(.bottom, keyboardInset)
                 .transition(.move(edge: .bottom))
             }
         }
@@ -135,6 +138,17 @@ struct ComposeView: View {
         }
     }
     
+    @available(iOS, obsoleted: 16.0)
+    private var keyboardInset: CGFloat {
+        if #unavailable(iOS 16.0),
+           UIDevice.current.userInterfaceIdiom == .pad,
+           keyboardReader.isVisible {
+            return 44
+        } else {
+            return 0
+        }
+    }
+    
     @ViewBuilder
     private var autocompleteSuggestions: some View {
         if let state = uiState.autocompleteState {
@@ -316,6 +330,26 @@ private struct GlobalFrameOutsideListPrefKey: PreferenceKey {
     }
 }
 
+@available(iOS, obsoleted: 16.0)
+private class KeyboardReader: ObservableObject {
+    @Published var isVisible = false
+    
+    init() {
+        NotificationCenter.default.addObserver(self, selector: #selector(willShow), name: UIResponder.keyboardWillShowNotification, object: nil)
+        NotificationCenter.default.addObserver(self, selector: #selector(willHide), name: UIResponder.keyboardWillHideNotification, object: nil)
+    }
+    
+    @objc func willShow(_ notification: Foundation.Notification) {
+        // when a hardware keyboard is connected, the height is very short, so we don't consider that being "visible"
+        let endFrame = notification.userInfo![UIResponder.keyboardFrameEndUserInfoKey] as! CGRect
+        isVisible = endFrame.height > 72
+    }
+    
+    @objc func willHide() {
+        isVisible = false
+    }
+}
+
 //struct ComposeView_Previews: PreviewProvider {
 //    static var previews: some View {
 //        ComposeView()