MailApp.m

- (void)MASetPreferencesController:(NSWindowController *)windowController {
    [self MASetPreferencesController:windowController];
    if(!windowController) {
        return;
    }
    
    NSToolbarItem *toolbarItem = [[NSToolbarItem alloc] initWithItemIdentifier:@"gpgmail"];
    toolbarItem.label = @"GPGMail";
    toolbarItem.image = [NSImage imageNamed:@"GPGMail"];
    NSToolbar *toolbar = [[windowController window] toolbar];
    BOOL alreadySetup = NO;
    for(NSToolbarItem *toolbarItem in [toolbar items]) {
        if([[toolbarItem itemIdentifier] isEqualToString:@"gpgmail"]) {
            alreadySetup = YES;
            break;
        }
    }
    if(!alreadySetup) {
        [toolbar insertItemWithItemIdentifier:@"gpgmail" atIndex:[[toolbar items] count]];
    }
    
    NSArray *topLevelObjects = nil;
    NSView *currentView = nil;
    [[GPGMailBundle bundle] loadNibNamed:@"GPGMailPreferences" owner:nil topLevelObjects:&topLevelObjects];
    for(id object in topLevelObjects) {
        if([object isKindOfClass:NSClassFromString(@"GMSpecialBox")]) {
            currentView = object;
            break;
        }
    }
    NSTabViewItem *item = [[NSTabViewItem alloc] initWithIdentifier:@"gpgmail"];
    NSViewController *GMPreferencesController = [[NSViewController alloc] init];
    GMPreferencesController.view = currentView;
    item.view = GMPreferencesController.view;
    NSTabViewController *controller = (NSTabViewController *)[windowController contentViewController];
    item.viewController = GMPreferencesController;
    [controller addTabViewItem:item];
    
    NSRect frame = [[windowController window] frame];
    float width = 0.0f;
    NSArray *subviews = [[[toolbar valueForKey:@"_toolbarView"] subviews][0] subviews];
    for (NSView *view in subviews) {
        width += view.frame.size.width;
    }
    // Add padding to fit them all.
    // Migt not be necessary to adjust the frame.
    frame.size = NSMakeSize(width > frame.size.width ? width : frame.size.width, frame.size.height);
    [[windowController window] setMinSize:frame.size];
    [[windowController window] setFrame:frame display:YES];
    // Fixing the width constraint is however necessary.
    [[[[[windowController window] contentView] widthAnchor] constraintEqualToConstant:width] setActive:YES];
}

MailTabViewController_GPGMail.m

- (NSToolbarItem *)MAToolbar:(NSToolbar *)toolbar
     itemForItemIdentifier:(NSString *)itemIdentifier
 willBeInsertedIntoToolbar:(BOOL)flag {
    if([[itemIdentifier lowercaseString] isEqualToString:@"gpgmail"]) {
        NSToolbarItem *toolbarItem = [[NSToolbarItem alloc] initWithItemIdentifier:@"gpgmail"];
        toolbarItem.label = @"GPGMail";
        toolbarItem.image = [NSImage imageNamed:@"GPGMail"];
        toolbarItem.target = self;
        toolbarItem.action = @selector(showGPGMailPreferences:);
        return toolbarItem;
    }
    
    id ret = [self MAToolbar:toolbar itemForItemIdentifier:itemIdentifier willBeInsertedIntoToolbar:flag];
    
    return ret;
}

- (void)showGPGMailPreferences:(id)item {
    NSWindowController *preferencesController = [(id)[NSClassFromString(@"MailApp") sharedApplication] preferencesController];
    [[(NSTabViewController *)[preferencesController contentViewController] tabView] selectTabViewItemWithIdentifier:[item itemIdentifier]];
}