Browser screen recorder cross-platform

Hi, I have an application that displays animations inside node-webkit, it is based on CHROME (chromium - v8) so maybe we can add it to our app. You need to create a native way to record a specific part of the screen in this case, the child window where the animation is running. This windows is always on top so you only need to be sure you are capturing/recording the animation as soon as the animation starts on a specific part of the screen. The animation is javascript based. We were trying to use VLC recorder but for some reason it is not starting or stopping at the same time when the animation is running. The processes are not starting at the same time. If you have a native solution to record the animation (on the child window) on Windows and Mac please apply. Thanks