Fix Compose reply scroll effect not working on iOS 13

Tusker/Screens/Compose/ComposeReplyView.swift

@@ -11,6 +11,7 @@ import SwiftUI
 struct ComposeReplyView: View {
     let status: StatusMO
     let stackPadding: CGFloat
+    let outerMinY: CGFloat
     
     @State private var contentHeight: CGFloat?
     
@@ -48,8 +49,11 @@ struct ComposeReplyView: View {
     }
     
     private func replyAvatarImage(geometry: GeometryProxy) -> some View {
-        let scrollOffset = geometry.frame(in: .named("outer")).minY - stackPadding
-        let offset = min(max(-scrollOffset, 0), geometry.size.height - 50 - 8)
+        // using named coordinate spaces produces an incorrect scroll offset on iOS 13,
+        // so simply compare the geometry inside and outside the scroll view in the global coordinate space
+        var scrollOffset = outerMinY - geometry.frame(in: .global).minY
+        scrollOffset += stackPadding
+        let offset = min(max(scrollOffset, 0), geometry.size.height - 50 - stackPadding)
         return ComposeAvatarImageView(url: status.account.avatar)
             .offset(x: 0, y: offset)
     }

Tusker/Screens/Compose/ComposeView.swift

@@ -57,14 +57,15 @@ struct ComposeView: View {
     
     var mostOfTheBody: some View {
         ZStack(alignment: .top) {
-            ScrollView(.vertical) {
-                mainStack
+            GeometryReader { (outer) in
+                ScrollView(.vertical) {
+                    mainStack(outerMinY: outer.frame(in: .global).minY)
+                }
             }
             
             // can't use SwiftUI.ProgressView because there's no UIProgressView.Style.bar equivalent, see FB8587149
             WrappedProgressView(value: postProgress, total: postTotalProgress)
         }
-        .coordinateSpace(name: "outer")
         .onAppear(perform: self.didAppear)
         .navigationBarTitle("Compose")
         .actionSheet(isPresented: $uiState.isShowingSaveDraftSheet, content: self.saveAndCloseSheet)
@@ -77,13 +78,14 @@ struct ComposeView: View {
         }
     }
     
-    var mainStack: some View {
+    func mainStack(outerMinY: CGFloat) -> some View {
         VStack(alignment: .leading, spacing: 8) {
             if let id = draft.inReplyToID,
                let status = mastodonController.persistentContainer.status(for: id) {
                 ComposeReplyView(
                     status: status,
-                    stackPadding: stackPadding
+                    stackPadding: stackPadding,
+                    outerMinY: outerMinY
                 )
             }