IOS-838: ConversationInputTextView cursor adjustments don't behave properly

At the moment, when trying to drag the input/selection cursor(s) in
the ConverationInputTextView, as soon as you reach the top or bottom
edge of a draft with >4 lines, you'll immediately scroll to the very top
or the very bottom.

This is because we're currently overriding the default implementation of
setContentOffset:animated: to always perform the adjustment without
animation. We do this because we don't want the animated content offset
adjustment to interfere with the growing the bounds of the text view.
This can cause the text to overshoot the updated bounds pre-iOS 13.

But we only grow the text view in response to the text changing. So
instead of disabling all content offset animations, we can get away with
just disabling content offset animations while the text is changing.

So this commit:
- Sets a flag between -textViewShouldChange... and -textViewDidChange:
- On iOS 13 and later, we'll always respect the animate argument of
  setContentOffset:animated:
- Pre-iOS 13, we'll ignore the animate parameter if the flag is set

Author: michelle-signal

Signal/src/ViewControllers/ConversationView/ConversationInputTextView.swift

@@ -27,6 +27,7 @@ class ConversationInputTextView: MentionTextView {
 
     var trimmedText: String { text.ows_stripped() }
     var untrimmedText: String { text }
+    private var textIsChanging = false
 
     required init() {
         super.init()
@@ -114,10 +115,22 @@ class ConversationInputTextView: MentionTextView {
 
     override func setContentOffset(_ contentOffset: CGPoint, animated: Bool) {
         // When creating new lines, contentOffset is animated, but because because
-        // we are simultaneously resizing the text view, this can cause the
-        // text in the textview to be "too high" in the text view.
-        // Solution is to disable animation for setting content offset.
-        super.setContentOffset(contentOffset, animated: false)
+        // we are simultaneously resizing the text view, on pre-iOS 13 this can
+        // cause the text in the textview to be "too high" in the text view.
+        // Solution is to disable animation for setting content offset between
+        // -textViewShouldChange... and -textViewDidChange.
+        //
+        // We can't unilaterally disable *all* animated scrolling because that breaks
+        // manipulation of the cursor in scrollable text. Animation is required to
+        // slow the text view scrolling down to human scale when the cursor reaches
+        // the top or bottom edge.
+        let shouldAnimate: Bool
+        if #available(iOS 13, *) {
+            shouldAnimate = animated
+        } else {
+            shouldAnimate = animated && !textIsChanging
+        }
+        super.setContentOffset(contentOffset, animated: shouldAnimate)
     }
 
     override var contentInset: UIEdgeInsets {
@@ -169,8 +182,14 @@ class ConversationInputTextView: MentionTextView {
 
     // MARK: - UITextViewDelegate
 
+    override func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
+        textIsChanging = true
+        return true
+    }
+
     override func textViewDidChange(_ textView: UITextView) {
         super.textViewDidChange(textView)
+        textIsChanging = false
 
         updatePlaceholderVisibility()
         updateTextContainerInset()

Signal/src/ViewControllers/ConversationView/ConversationInputToolbar.m

@@ -27,8 +27,6 @@ typedef NS_CLOSED_ENUM(NSUInteger, VoiceMemoRecordingState){
 
 typedef NS_CLOSED_ENUM(NSUInteger, KeyboardType) { KeyboardType_System, KeyboardType_Sticker, KeyboardType_Attachment };
 
-static void *kConversationInputTextViewObservingContext = &kConversationInputTextViewObservingContext;
-
 const CGFloat kMinTextViewHeight = 36;
 const CGFloat kMinToolbarItemHeight = 44;
 const CGFloat kMaxTextViewHeight = 98;