Constrain certain dimensions while resizing groups of objects

I'm creating a mockup in Adobe Illustrator. I have a mockup of the Yosemite window (with the close, minimize, and fullscreen buttons) and some stuff in the main content area. The annoying this is that if I resize the mockup, the Yosemite buttons and the titlebar area get fatter. Is there a way to constrain the proportions, so that when I resize the entire screen, the titlebar will only resize in the x direction, and not in the y direction?

Here's a visual example of what I'm talking about:

Now when I resize, I get this undesirable behaviour:

The only way I've found to get around is to manually resize the content area and the titlebar area individually. Is there a better way to do this? Thanks!