Bar Properties dialog box - Groups do not stretch
Dimmed unless Button stretching is set to Extends.
You can create navigation bar buttons that stretch as you change the amount of text on them. You might not want all objects to stretch and this option marks groups as non-stretching. (A group can consist of a single object.) Thus you can group objects you want as non-stretching and then select this option.
Note:
The Button Stretching option controls whether the button stretches as you change the text. This option just controls stretching of groups.