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A general description is here.
configure will be able to find a usable libtiff isn some rare circumstances where it did not previously (where libtiff needed to be linked explicitly against -ljpeg).
[g]sub() could use excessive stack space when used with a very long vector of non-ASCII data.
X11() and x11() gain additional arguments to be more similar to the Unix X11() device.
'Rcmd open' now works as documented: previously 'Rcmd open.exe' was required.
cat(x, file=) in Rgui tried to interpret UTF-8-encoded strings for the console rather than converting them to the native encoding.