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On Mac, the back and forward buttons show an empty rectange instead of a grayed-out arrow when they are disabled. The reason is a Tk bug on Mac that causes disabled images not to draw correctly (not to draw at all, that is); see <https://groups.google.com/forum/?fromgroups=#!topic/comp.lang.tcl/V-nW1JBq0eU>. To work around this, we explicitly provide gray images for the disabled state; I think this looks better than the default stipple effect that you get on Windows as well, but that may be a matter of taste. Signed-off-by: Stefan Haller <stefan@haller-berlin.de> Signed-off-by: Paul Mackerras <paulus@samba.org>