descender, and {\it externalLeading} is any extra vertical space added
to the font by the font designer (usually is zero).
-The optional parameter {\it font} specifies an alternative
-to the currently selected font: but note that this does not
-yet work under Windows, so you need to set a font for
-the device context first.
+If the optional parameter {\it font} is specified and valid, then it is used
+for the text extent calculation. Otherwise the currently selected font is.
See also \helpref{wxFont}{wxfont}, \helpref{wxDC::SetFont}{wxdcsetfont}.