How to avoid reconnecting an already paired device via WebBluetooth on every page reload.

I can confirm your experience on Windows 10/CSR8510 (Win10 Bluetooth Driver)/Chrome 90:

• Go to https://googlechrome.github.io/samples/web-bluetooth/watch-advertisements-and-connect-async-await.html
  
• Request and pair your Espruino
  
• Close/Reopen Chrome and go to the same site
  
• Press "Connect"
  
• Wait (up to 10 seconds in my 5 runs)
  
  

Result:

Getting existing permitted Bluetooth devices...
> Got 1 Bluetooth devices.
Watching advertisements from "Puck.js c699"...
> Received advertisement from "Puck.js c699"...
Connecting to GATT Server from "Puck.js c699"...
> Bluetooth device "Puck.js c699 connected.

On macOS I don't get a reconnect because no advertisement packets are received unless I explicitly trigger a scan or request a device.