Damn.  This was working at one point.

Created attachment 296398 [details] [review]
Set the focus on a composer detached using the detach button

Actually, I'm no longer so sure that this used to work.  I didn't read 
carefully enough to notice that you said this only happens with the 
detach button, not with the accelerator.  I'm not sure we ever had it 
working with the detach button, for the reason you note -- once we 
realize that the detach button has been pressed, it's too late to figure 
out what used to have the focus.

But I think we can get it right 99% of the time by calling the 
set_focus() method, which puts the focus in the To box if it's empty, or 
the Subject box if it's empty, or the editor.  I don't know if we used 
to get that called when the detach button was pressed, but it isn't 
happening any more.  I put in a check if the focused widget is visible 
before we try to refocus it.  (Actually, we look at the widget's parent, 
since it's the box containing the detach button that we hide.)  The 
alternative to this would be to keep track of where the focus used to 
be, but I don't know that that's worth all the extra work.

I also took this opportunity to tighten up the focus code.  I don't know 
why I had the containers returning the focused widget, but we can figure 
it out easily directly in the composer widget itself.  If you like, I 
can split that out into a separate patch.

Review of attachment 296398 [details] [review]:

Since this is a fairly simple patch, let's get the code tightening in this commit as well.  Glad to see this was so straightforward.

Attachment 296398 [details] pushed as 9ec9913 - Set the focus on a composer detached using the detach button