[Feature Request] Add "Responsive" width viewport

[caseymhunt]

It would be helpful to have access to a "responsive" (user-adjustable) width viewport, in addition to the configured widths in the .config file.

Ideally this "responsive" width would be appended to the list of supplied widths and would appear as an additional optional window width in Playroom.

[roddds]

You can get around that by adding 'calc(100vw - 120px)' to the list of widths, and then resizing the browser window.

[caseymhunt]

You can get around that by adding 'calc(100vw - 120px)' to the list of widths, and then resizing the browser window.

That's a great workaround- a bit ugly but neat! thank you.

It would be great to be able to mask the css formula with a human-readable name, though.

[askoufis]

Not sure if we'd implement this directly, but with #222 it makes the workaround more viable since you'd be able to rename the width to responsive for example.