Ensure that, when pasting chunks, a surrogate pair does not get cut in half. Issue 5040

sources/NSStringITerm.h

@@ -248,6 +248,9 @@ int decode_utf8_char(const unsigned char * restrict datap,
 // Contains only digits?
 - (BOOL)isNumeric;
+// Modify the range's endpoint to not sever a surrogate pair.
+- (NSRange)makeRangeSafe:(NSRange)range;
+
 @end
 @interface NSMutableString (iTerm)

sources/NSStringITerm.m

@@ -1615,6 +1615,21 @@ static TECObjectRef CreateTECConverterForUTF8Variants(TextEncodingVariant varian
     return range.location == NSNotFound;
 }
+- (NSRange)makeRangeSafe:(NSRange)range {
+    if (range.location == NSNotFound || range.length == 0) {
+        return range;
+    }
+    unichar lastCharacter = [self characterAtIndex:NSMaxRange(range) - 1];
+    if (CFStringIsSurrogateHighCharacter(lastCharacter)) {
+        if (NSMaxRange(range) == self.length) {
+            range.length -= 1;
+        } else {
+            range.length += 1;
+        }
+    }
+    return range;
+}
+
 @end
 @implementation NSMutableString (iTerm)

sources/iTermPasteHelper.m

@@ -394,6 +394,7 @@ const int kNumberOfSpacesPerTabNoConversion = -1;
                 block = YES;
             }
         }
+        range = [_buffer makeRangeSafe:range];
         [_delegate pasteHelperWriteString:[_buffer substringWithRange:range]];
     }
     [_buffer replaceCharactersInRange:range withString:@""];