That currently works as well. You can open Compose in a new tab with Control-click for Win/Linux or Cmd-click for Mac.

I rarely see the appeal of tabs, so generally use windows. They are great as temporary bookmarks, but not for working with multiple sources in parallel. I'm confused that this is not more common, other than that more and more sites break it. Similar to the "new" Google compose window, and unlike tabs, the main advantage is that you can see both source and compose at once without needed to switch back and forth.

I'll turn the question back at you: why don't people use new windows? Is it just backlash from terribly designed sites that automatically pop up new windows for everything?