Yes, it is a layout problem. Every component has a layout mode that governs how the component will resize when the client resizes. Your tab strip is set to Relative mode which means it will resize relative to the new width and height of the window. So, what you need to do is change the layout mode to anchored top left and bottom. Make the tab strip the full height of the window in the designer and when it resizes the height will always be the height of the window.
To change the layout mode right click on the tab strip and select layout mode.