Deployed MakeCode maintains WebUSB connection with inactive tab, local `pxt serve` doesn't

Testing locally with macOS and the default configuration:

git clone https://github.com/microsoft/pxt-microbit.git
cd pxt-microbit
npm install
npx pxt serve

With a simple programme sends some serial data: https://makecode.microbit.org/_YxU1cDhkjYs3

As soon as the micro:bit is WebUSB connected we can see the orange DAPLink LED blinking every second or so, which indicates it's sending the "hello" string. This is as expected, so all good so far.

However, a this point, there is a difference between https://makecode.microbit.org and serving locally:

• Online Makecode: When the tab is switched or the minimised, the WebUSB connection is still active and we can see the orangle LED still blinking (sending serial data) once per second.
• Local pxt serve: As soon as the tab is inactive the orange LED stops blinking. When the MakeCode tab is on focus again the micro:bit resets and the orange LED starts blinking again.

So it looks like the online deployment maintain the WebUSB connection when the tab is inactive, and the local serve does not, so it reconnects when the tab comes back on focus.

I noticed this when looking at why the micro:bit was resetting twice on WebUSB connections (raised PR https://github.com/microsoft/pxt-microbit/pull/4953 as part of that).

Is this on purpose? Is there a specific setting to switch this behaviour?

Start with the local npx pxt serve entry point and reproduce the WebUSB behavior using the setup and serial-data program described in the issue. Compare the inactive-tab behavior with deployed MakeCode, then determine whether a setting explains the difference or whether a code change is needed; done means the intended behavior and any required change are established.