We're getting closer to ideal sizing of JButtons under GTK L&F
after the fixes for:
6489585: GTK L&F: buttons, checkboxes, and radiobuttons are sized incorrectly
6479305: GTK L&F: more toolbar issues

But while investigating 6479305, it was discovered that we are always leaving
enough space for the "default-border", even though most JButtons are not actually
the "default button" (i.e. isDefaultButton() returns false). Ideally we would
only include room for the "default-border" if the button is in fact the default one.
Unfortunately, the ideal solution is complicated. See comments section for an
email thread on the topic.