@Claude31 Sorry for the late reply. I select the Xib file that is in the project and then I select the NSView drop down menu I can't do the following things. When selecting New Alighment Contraint, I can't add Horizontally in Container, Vertically in Container. I can't enter any values and Add Constraint. Thanks.

Thank you for your response. I'll give it a try. Thanks.

Where do you see NSThemeFrame is in the hierarchy In Xcode 10.1 while debugging, I click 'Debug View Heirarchy' and the Heirarchy shows that NSThemeFrame is in between NSWindow and NSViewController. NSThemeFrame is not a VirtualBox object. When I select NSThemeFrame and look at the details, it shows as below. Object &#9; Class Name:&#9;NSThemeFrame &#9; Address:&#9;0x101a13660 View &#9; Wants Layer:&#9;Yes &#9; Layer:&#9;<_NSViewBackingLayer: 0x600000c89620> &#9; Focus Ring:&#9;Default &#9; Visibility:&#9;Not Opaque &#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9;Not Hidden &#9; Alpha:&#9;1 &#9; Flipped: No &#9; Drawing:&#9;Can Draw Concurrently Off &#9; Identifier:&#9;<Null> &#9; Toop Tip:&#9;<Null> &#9; Appearance:&#9;<Null> &#9; Effective:&#9;NSAppearanceNameAqua Accessibility &#9; Descendant:&#9;<Null> &#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9;&#9; Not Hidden &#9; Label:&#9;<Null> &#9; Description:&#9;<NSThemeFrame: 0x101a13660> Hierarchy &#9; NSThemeFrame &#9; NSTitledFrame &#9; NSFrameView &#9; NSView &#9; NSResponder &#9; NSObject Backtrace &#9; Malloc stack logging is not enabled for this process.