Thanks for posting here in the Google Nest Community. Kindly help us better understand what’s going on. You said that this happens when you restart your computer; does that mean your computer is connected to the TV via HDMI (example: HDMI1)? And as it restarts, the TV switches to another HDMI input where the Chromecast device is connected (example: HDMI2). What Chromecast device do you have connected to your TV? What do you see on the screen when it switches to Chromecast? It would help us more if you could share more details.
