Fix crash with CJK keyboard with emoji at end of text field (#42540)

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 #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

@@ -1658,25 +1658,6 @@ - (CGRect)firstRectForRange:(UITextRange*)range {
   return CGRectZero;
 }
 
-- (BOOL)isRTLAtPosition:(NSUInteger)position {
-  // _selectionRects is sorted by position already.
-  // We can use binary search.
-  NSInteger min = 0;
-  NSInteger max = [_selectionRects count];
-  while (min <= max) {
-    const NSUInteger mid = min + (max - min) / 2;
-    FlutterTextSelectionRect* rect = _selectionRects[mid];
-    if (rect.position > position) {
-      max = mid - 1;
-    } else if (rect.position == position) {
-      return rect.isRTL;
-    } else {
-      min = mid + 1;
-    }
-  }
-  return NO;
-}
-
 - (CGRect)caretRectForPosition:(UITextPosition*)position {
   NSInteger index = ((FlutterTextPosition*)position).index;
   UITextStorageDirection affinity = ((FlutterTextPosition*)position).affinity;
@@ -1699,7 +1680,8 @@ - (CGRect)caretRectForPosition:(UITextPosition*)position {
     CGRect characterAfterCaret = rects[0].rect;
     // Return a zero-width rectangle along the upstream edge of the character after the caret
     // position.
-    if ([self isRTLAtPosition:index]) {
+    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 {
@@ -1712,7 +1694,8 @@ - (CGRect)caretRectForPosition:(UITextPosition*)position {
     CGRect characterAfterCaret = rects[1].rect;
     // Return a zero-width rectangle along the upstream edge of the character after the caret
     // position.
-    if ([self isRTLAtPosition:index]) {
+    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 {
@@ -1727,7 +1710,8 @@ - (CGRect)caretRectForPosition:(UITextPosition*)position {
   // 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 ([self isRTLAtPosition:index - 1]) {
+  if ([rects[0] isKindOfClass:[FlutterTextSelectionRect class]] &&
+      ((FlutterTextSelectionRect*)rects[0]).isRTL) {
     return CGRectMake(characterBeforeCaret.origin.x, characterBeforeCaret.origin.y, 0,
                       characterBeforeCaret.size.height);
   } else {

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

@@ -1558,6 +1558,29 @@ - (void)testClosestPositionToPointWithinRange {
       ((FlutterTextPosition*)[inputView closestPositionToPoint:point withinRange:range]).affinity);
 }
 
+- (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 {