Fix compose reply avatar scroll effect not working on iOS 15

Tusker/Screens/Compose/ComposeReplyView.swift

@@ -52,9 +52,20 @@ struct ComposeReplyView: View {
     private func replyAvatarImage(geometry: GeometryProxy) -> some View {
         // 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)
+        let scrollOffset = outerMinY - geometry.frame(in: .global).minY
+        
+        // add stackPadding so that the image is always at least stackPadding away from the top
+        var offset = scrollOffset + stackPadding
+        
+        // offset can never be less than 0 (i.e., above the top of the in-reply-to content)
+        offset = max(offset, 0)
+        
+        // subtract 50, because we care about where the bottom of the view is but the offset is relative to the top of the view
+        let maxOffset = (contentHeight ?? 0) - 50
+        
+        // once you scroll past the in-reply-to-content, the bottom of the avatar should be pinned to the bottom of the content
+        offset = min(offset, maxOffset)
+        
         return ComposeAvatarImageView(url: status.account.avatar)
             .frame(width: 50, height: 50)
             .cornerRadius(preferences.avatarStyle.cornerRadiusFraction * 50)