This is a meta bug for tracking platform changes that would enable Firefox to render persistent UI that is transparent and placed directly over the content area. Hypothetical examples include a transparent find bar that allows the page to partially display beneath it, or an opaque find bar that only covers 1/3 of the way across the bottom of the screen. (no idea what component this should be in, since it involves combining the chrome and content view frame hierarchies, please feel free to move it to a more appropriate component).
For a little more context here: intended usage is likely going to be a small set of navigational controls (back/forward/reload) in the upper left hand corner of the otherwise chromeless user created application tabs. I used the find bar example since it's a UI that more people are familiar with. I'll try to get some mockups put together of the app tab UI. Ultimately we will likely end up using this capability for a variety of different interfaces.
This can be used to open sidebar for a moment and do not shift page content.