browser.windows.create() always creates a window with a title bar. This is slow.
The previous non-webext add-on could use window.open() to open a panel with no title bar.
I haven’t been able to get window.open() to work in the new webext version.
Is it possible or will it be possible in the future to create a window without a title bar?
WebExtensions can generally only do things web pages can plus what is (quite well) documented on AMO.
It is possible that Mozilla will add a frameless window type, if it is requested. I think it would be nice to have, but could imagine that one of their concerns is that you could use such windows to cover up parts of the UI. And you could effectively use them as a browserAction panel that can be opened at any time, which is explicitly not possible with the normal browserAction panel.
A frameless panel was a standard part of the previous add-on SDK so they shouldn’t be too concerned.
I wanted that too, but Mozilla said they won’t make it possible. I’ve started work on a floating panel API to solve my needs, which was discussed with the Mozilla folks to be okay, but it has been some time since anything has moved on this topic:
https://bugzilla.mozilla.org/show_bug.cgi?id=1364404
I would be fine if there were a fixed border around the popup so that it can’t be mistaken for a part of the page. An icon would be OK if it didn’t take too much space. On Linux, if there is a title bar then the window manager gets involved and it takes so much longer to pop up and usually the position is arbitrary.