If I'm understanding the CDDL spec correctly, and inferring the intended definition of browser.SetClientWindowStateParameters correctly, then browser.SetClientWindowStateParameters is defined incorrectly; it uses // when it meant to use /.

That is, // is just a weakly-binding /, allowing you to define groups inline without having to abstract them to types. The current definition of browser.SetClientWindowStateParameters is:

browser.SetClientWindowStateParameters = {
  clientWindow: browser.ClientWindow,
  browser.ClientWindowNamedState // browser.ClientWindowRectState
}

This is, afaict, equivalent to:

browser.SetClientWindowStateParameters = {
  (clientWindow: browser.ClientWindow, browser.ClientWindowNamedState) // 
  browser.ClientWindowRectState
}

That is, the first two items form a group, and the third item forms an alternative group. But I suspect what was intended was for clientWindow to always be present, and then have a choice between browser.ClientWindowNamedState and browser.ClientWindowRectState. To do that, you would need to have written:

browser.SetClientWindowStateParameters = {
  clientWindow: browser.ClientWindow,
  browser.ClientWindowNamedState / browser.ClientWindowRectState
}