Q: How can I selectively mirror with Apple TV? Instead of mirroring my mac's entire display, can I just mirror a browser window, movie, app, etc. while I continue working?

No, you can't do that, if you have any suggestions that you think might enhance the Apple TV you can send Apple your feedback here

You can, however, AirPlay a video using the AirPlay icon that appears in the player, not the one in the main menu. Mirroring applies to the full screen and is unlikely to change, since it's implemented in the graphics chip, where the video data is taken from the frame buffer and converted to H.264 video for transmission.

Yes, for clarification, you cannot be selective about what you send to your Apple TV, so you can't send your browser window or an application window for example but you can send video and audio streams that you may get via a web page.

You will be able to do this when you see the airplay icon in suitable content.

This isn't actually airplay mirroring and therefore is subject to the hardware restriction that the mirroring feature is.

Also, instead of mirroring, you can extend the desktop to the ATV screen, see Use AirPlay to display video from your Mac on an HDTV - Apple Support

This makes ATV a second acreen. You can drag windows to it and they will display there.