Fix crash with CJK keyboard with emoji at end of text field (flutter#42539)

The `isRTLAtPosition` method had a bug, it used `NSInteger max = [_selectionRects count]` instead of `NSInteger max = [_selectionRects count] - 1`. But I realized we don't even need the function any more, it was used in a few places in previous iterations of flutter#36643, but in the only place remaining, we actually already have the selection rect and don't need to search for it by position.

Btw as an explanation of the crash, I guess there is some mismatch between code point and character count somewhere. UIKit was asking for `caretRectForPosition:2` when we only had 1 character. This could have only crashed when floating cursor selection was used, but actually when switching to CJK keyboard, UIKit turns out to use `caretRectForPosition` to calculate something about the composing rect.

Fixes flutter/flutter#128031

Author: moffatman

shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -1600,22 +1600,65 @@ - (CGRect)firstRectForRange:(UITextRange*)range {
 }
 
 - (CGRect)caretRectForPosition:(UITextPosition*)position {
-  // TODO(cbracken) Implement.
-
-  // As of iOS 14.4, this call is used by iOS's
-  // _UIKeyboardTextSelectionController to determine the position
-  // of the floating cursor when the user force touches the space
-  // bar to initiate floating cursor.
-  //
-  // It is recommended to return a value that's roughly the
-  // center of kSpacePanBounds to make sure the floating cursor
-  // has ample space in all directions and does not hit kSpacePanBounds.
-  // See the comments in beginFloatingCursorAtPoint.
-  return CGRectZero;
-}
+  NSInteger index = ((FlutterTextPosition*)position).index;
+  UITextStorageDirection affinity = ((FlutterTextPosition*)position).affinity;
+  // Get the selectionRect of the characters before and after the requested caret position.
+  NSArray<UITextSelectionRect*>* rects = [self
+      selectionRectsForRange:[FlutterTextRange
+                                 rangeWithNSRange:fml::RangeForCharactersInRange(
+                                                      self.text,
+                                                      NSMakeRange(
+                                                          MAX(0, index - 1),
+                                                          (index >= (NSInteger)self.text.length)
+                                                              ? 1
+                                                              : 2))]];
+  if (rects.count == 0) {
+    return CGRectZero;
+  }
+  if (index == 0) {
+    // There is no character before the caret, so this will be the bounds of the character after the
+    // caret position.
+    CGRect characterAfterCaret = rects[0].rect;
+    // Return a zero-width rectangle along the upstream edge of the character after the caret
+    // position.
+    if ([rects[0] isKindOfClass:[FlutterTextSelectionRect class]] &&
+        ((FlutterTextSelectionRect*)rects[0]).isRTL) {
+      return CGRectMake(characterAfterCaret.origin.x + characterAfterCaret.size.width,
+                        characterAfterCaret.origin.y, 0, characterAfterCaret.size.height);
+    } else {
+      return CGRectMake(characterAfterCaret.origin.x, characterAfterCaret.origin.y, 0,
+                        characterAfterCaret.size.height);
+    }
+  } else if (rects.count == 2 && affinity == UITextStorageDirectionForward) {
+    // There are characters before and after the caret, with forward direction affinity.
+    // It's better to use the character after the caret.
+    CGRect characterAfterCaret = rects[1].rect;
+    // Return a zero-width rectangle along the upstream edge of the character after the caret
+    // position.
+    if ([rects[1] isKindOfClass:[FlutterTextSelectionRect class]] &&
+        ((FlutterTextSelectionRect*)rects[1]).isRTL) {
+      return CGRectMake(characterAfterCaret.origin.x + characterAfterCaret.size.width,
+                        characterAfterCaret.origin.y, 0, characterAfterCaret.size.height);
+    } else {
+      return CGRectMake(characterAfterCaret.origin.x, characterAfterCaret.origin.y, 0,
+                        characterAfterCaret.size.height);
+    }
+  }
 
-- (CGRect)bounds {
-  return _isFloatingCursorActive ? kSpacePanBounds : super.bounds;
+  // Covers 2 remaining cases:
+  // 1. there are characters before and after the caret, with backward direction affinity.
+  // 2. there is only 1 character before the caret (caret is at the end of text).
+  // For both cases, return a zero-width rectangle along the downstream edge of the character
+  // before the caret position.
+  CGRect characterBeforeCaret = rects[0].rect;
+  if ([rects[0] isKindOfClass:[FlutterTextSelectionRect class]] &&
+      ((FlutterTextSelectionRect*)rects[0]).isRTL) {
+    return CGRectMake(characterBeforeCaret.origin.x, characterBeforeCaret.origin.y, 0,
+                      characterBeforeCaret.size.height);
+  } else {
+    return CGRectMake(characterBeforeCaret.origin.x + characterBeforeCaret.size.width,
+                      characterBeforeCaret.origin.y, 0, characterBeforeCaret.size.height);
+  }
 }
 
 - (UITextPosition*)closestPositionToPoint:(CGPoint)point {

shell/platform/darwin/ios/framework/Source/FlutterTextInputPluginTest.mm

@@ -1429,6 +1429,29 @@ - (void)testClosestPositionToPointWithinRange {
       1U, ((FlutterTextPosition*)[inputView closestPositionToPoint:point withinRange:range]).index);
 }
 
+- (void)testClosestPositionToPointWithPartialSelectionRects {
+  FlutterTextInputView* inputView = [[FlutterTextInputView alloc] initWithOwner:textInputPlugin];
+  [inputView setTextInputState:@{@"text" : @"COMPOSING"}];
+
+  [inputView setSelectionRects:@[ [FlutterTextSelectionRect
+                                   selectionRectWithRect:CGRectMake(0, 0, 100, 100)
+                                                position:0U] ]];
+  // Asking with a position at the end of selection rects should give you the trailing edge of
+  // the last rect.
+  XCTAssertTrue(CGRectEqualToRect(
+      [inputView caretRectForPosition:[FlutterTextPosition
+                                          positionWithIndex:1
+                                                   affinity:UITextStorageDirectionForward]],
+      CGRectMake(100, 0, 0, 100)));
+  // Asking with a position beyond the end of selection rects should return CGRectZero without
+  // crashing.
+  XCTAssertTrue(CGRectEqualToRect(
+      [inputView caretRectForPosition:[FlutterTextPosition
+                                          positionWithIndex:2
+                                                   affinity:UITextStorageDirectionForward]],
+      CGRectZero));
+}
+
 #pragma mark - Floating Cursor - Tests
 
 - (void)testFloatingCursorDoesNotThrow {