ButtonExpandable is an option for ButtonBox which specifies whether the button should expand to fill any GridBox position in which it appears.
The default setting is ButtonExpandable->True.
This setting is usually used for all buttons that appear in palettes.
With ButtonExpandable->False the size of a button is determined purely by its contents, independent of its environment.
With ButtonExpandable->False, gutters will often be left between buttons in a GridBox.
See The Mathematica Book: Section 2.8.11.
See also: ButtonMargins, ButtonMinHeight, TextJustification.
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