I figured out the real problem. Responsive Design Mode (RDM) works just fine between applications and tabs.
The problem is that it does not persist when using Hot Module Reloading during development.
See the attached video (that didn't capture my audio 😡).
You'll see I start in RDM, switch to Firefox, come back to Orion and I'm still in RDM.
However, when I save a file in Visual Studio Code that triggers RDM, Astro reloads the page as usual, but loses RDM.
This means I have to switch back to RDM after every code change. This is too cumbersome.