ipad - 在 UIBarButtomItem 上呈现的 iPad UIPopover 覆盖了 BarButtomItem？

Question

我有 UIBarButtomItem 按下时显示弹出框：

//add help button
self.navigationItem.leftBarButtonItem = [[[UIBarButtonItem alloc] initWithTitle:NSLocalizedString(@"Help", @"Help") style:UIBarButtonItemStylePlain target:self action:@selector(showInfoBubble:)] autorelease];
infoBubblePopOverVisible = NO;
self.infoBubblePopOverController = nil;

这是显示信息气泡：

- (void) showInfoBubble: (id) sender {
[self dismissPopoverControllerExplicitly];
    
if (self.infoBubblePopOverController == nil) {
    InfoBubbleViewController *controller = [[InfoBubbleViewController alloc] initWithNibName:@"InfoBubbleViewController" bundle:nil];
    UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:controller];
    self.infoBubblePopOverController = [[UIPopoverController alloc] initWithContentViewController:navigationController];
    [controller release];
    [navigationController release];
}

//present popOverController
[self.infoBubblePopOverController presentPopoverFromBarButtonItem:self.navigationItem.leftBarButtonItem permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
infoBubblePopOverVisible = YES;

}

但是当呈现 popover 时，呈现部分覆盖 barbuttomitem：

我怎样才能解决这个问题？