I don't know Pixelmator but I think there's an option for that. In Preferences-&...

Avshalom on May 3, 2012 | parent | context | favorite | on: GIMP 2.8 (Stable) Finally Available For Download

I don't know Pixelmator but I think there's an option for that. In Preferences->window management you can change it from regular window to utility window, then it's just a question of whether your WM cares.