allow DevTools to be pinned to the browser window (bottom or right)

[cvan]

By default, Firefox opens the DevTools as a pane fixed to the bottom of the active browser window.

You can also dock the DevTools to the side:

And to the bottom again:

Or detach the DevTools:

And by default, qbrt opens the DevTools as a separate detached window:

For comparison, Electron does by default open the DevTools to the right side of the browser window.

To do this with the qbrt, the shell would be need to maintain an <iframe> that's positioned to the bottom, where the DevTools pane would be embedded. Am I correct in understanding that?

[mykmelez]

For comparison, Electron does by default open the DevTools to the right side of the browser window.

Indeed! I didn't do this earlier for a couple of reasons: primarily, because the shell was originally implemented in HTML, which doesn't support in-window DevTools; and secondarily, because I based the implementation on the one in Positron in order to get it working quickly, and that implementation didn't support in-window DevTools.

To do this with the qbrt, the shell would be need to maintain an <iframe> that's positioned to the bottom, where the DevTools pane would be embedded. Am I correct in understanding that?

Yes, I think that's correct, although I'm having trouble finding the canonical example in Firefox code. Here's an example of programmatically adding such an iframe and loading DevTools into it in a test:

http://searchfox.org/mozilla-central/rev/a798ee4fc323f9387b7576dbed177859d29d09b7/devtools/client/framework/test/browser_toolbox_target.js#21-37

However, I expect there to be a persistent iframe for DevTools in the browser window XUL (or perhaps the XBL bindings that implement the tabbrowser and browser interfaces), which the DevTools code simply hides/shows and moves around as needed. Perhaps that's not the case (or no longer the case), however.