Transparency does not work on themes that specify both focused and inactive pixmaps

Hi. I'm trying to make a theme with completely opaque window decor for
active windows, and transparency for certain parts of the inactive window
decor. I do not want the whole window to become transparent, only certain
frame parts. So I've made a theme that specifies both a 'normal' and
'focused' pixmap for each frame part, with some normal parts getting a
transparent pixmap. Unfortunately, glitching and remenants of any opaque
pixmaps used appear where a completely transparent effect was intended.

Here is an existing theme that suffers from the same problem:
http://sawfish.wikia.com/wiki/HackerPurpleS

In the screenshot you can see that the active window is meant to have a
thicker purple bar to show that the window is selected, whereas the
inactive window uses transparency to make a thinner bar. But test this
theme out and you will notice that graphical glitching appears where the
transparency should happen (a black bar appears). I've tested this against
multiple graphics cards and multiple versions of sawfish and xorg, with and
without compositing.

Is there a workaround for this that will allow me to make a theme where an
opaque pixmap is used on the active window, and a transparent pixmap is
used for "gaps" on the inactive window?

Thanks for reading.