Ensure the cursor remains visible when composing posts

Tusker/Screens/Compose/MainComposeTextView.swift

@@ -176,6 +176,7 @@ struct MainComposeWrappedTextView: UIViewRepresentable {
         var text: Binding<String>
         var didChange: (UITextView) -> Void
         var uiState: ComposeUIState
+        private var caretScrollPositionAnimator: UIViewPropertyAnimator?
         
         init(text: Binding<String>, uiState: ComposeUIState, didChange: @escaping (UITextView) -> Void) {
             self.text = text
@@ -186,6 +187,54 @@ struct MainComposeWrappedTextView: UIViewRepresentable {
         func textViewDidChange(_ textView: UITextView) {
             text.wrappedValue = textView.text
             didChange(textView)
+            
+            ensureCursorVisible()
+        }
+        
+        private func ensureCursorVisible() {
+            guard let textView = textView,
+                  textView.isFirstResponder,
+                  let range = textView.selectedTextRange,
+                  let scrollView = findParentScrollView() else {
+                return
+            }
+            
+            // We use a UIViewProperty animator to change the scroll view position so that we can store the currently
+            // running one on the Coordinator. This allows us to cancel the running one, preventing multiple animations
+            // from attempting to change the scroll view offset simultaneously, causing it to jitter around. This can
+            // happen if the user is pressing return and quickly creating many new lines.
+            
+            if let existing = caretScrollPositionAnimator {
+                existing.stopAnimation(true)
+            }
+            
+            let cursorRect = textView.caretRect(for: range.start)
+            var rectToMakeVisible = textView.convert(cursorRect, to: scrollView)
+            
+            // move Y position of the rect that will be made visible down by the cursor's height so that there's
+            // some space between the bottom of the line of text being edited and the top of the keyboard
+            rectToMakeVisible.origin.y += cursorRect.height
+            
+            let animator = UIViewPropertyAnimator(duration: 0.1, curve: .linear) {
+                scrollView.scrollRectToVisible(rectToMakeVisible, animated: false)
+            }
+            self.caretScrollPositionAnimator = animator
+            animator.startAnimation()
+        }
+        
+        private func findParentScrollView() -> UIScrollView? {
+            guard let textView = textView else { return nil }
+            
+            var current: UIView = textView
+            while let superview = current.superview {
+                if let scrollView = superview as? UIScrollView {
+                    return scrollView
+                } else {
+                    current = superview
+                }
+            }
+            
+            return nil
         }
         
         @objc func formatButtonPressed(_ sender: UIBarButtonItem) {