Afterward, Mikael commented that the app should really be coded not to have borders in the first place.
Me: You can do that?
Turns out it's not a standard ICCCM request, but one that mwm introduced, MWM_HINTS_DECORATIONS. Mikael pointed me to the urxvt source as an example of an app that uses it.
My own need was more modest: my little moonroot Xlib program that draws the moon at approximately its current phase. Since the code is a lot simpler than urxvt, perhaps the new version, moonroot 0.4, will be useful as an example for someone (it's also an example of how to use the X Shape extension for making non-rectangular windows).
[ 12:06 Oct 20, 2008 More programming | permalink to this entry | ]