How to remove window decoration under OSX?

Solution 1:

It's easy to do with simbl: Look for https://github.com/ksuther/SimulatorBorderKiller for a sample of the simbl plugin that disable all these things for the iPhone simulator.

If you need it for a special application, just ask it and I'll send you the compiled SIMBL plugin

Solution 2:

Only the frontmost window has this huge shadow, it's smaller for background windows. So you can try screenshotting non-focused windows (easy if you activate e.g. Mail and close all its windows, so no window has focus). However you would have to live with the deactivated, washed out look or manually merge both deactivated and active window screenshots.