I've configured Compiz to use gtk-window-decorator so that it may apply my Metacity theme for the titlebars. With this decorator, I've noticed that Compiz renders slightly blurry titlebar fonts when a window is not maximized. Once the window is maximized, sharp fonts are rendered.
When a window is not maximized, the font has a lighter color and seems to lack subpixel hinting or anti-aliasing, hence the slightly blurry appearance. I zoomed in on the font when the window was maximized and observed bluish lines around the characters. When a window was not maximized, the zoomed-in characters didn't have the bluish lines.
I've tested and seen that the Marco window manager (forked from Metacity for the MATE desktop environment) and the Emerald decorator (used with Compiz) both render sharp titlebar fonts regardless of whether a window is maximized or not.
Could this be caused by a lack of subpixel hinting or anti-aliasing in gtk-window-decorator when a window is not maximized?
The image in the below link shows the slight difference in font rendering when Compiz is used with gtk-window-decorator. Firefox's window was maximized, with sharp text, while VLC's window was not maximized, with slightly blurry text.
Upon switching to Marco window manager, both texts are sharp.
When the "Compiz with gtk-window-decorator" setup is zoomed-in, blue lines around the maximized Firefox window titlebar text are visible (it looks like subpixel rendering to me). The blue lines are completely lacking in the un-maximized VLC window titlebar text.
Testing with different Metacity themes has the same slightly blurry result. It seems to be a window decorator issue.