11.04 can't drag-over taskbar window to set focus

Since upgrading to 11.04 (and changing from Unity to the old desktop style), I can no longer drag an item (such as an email from Thunderbird) over a window in my task bar to bring that window into focus.

I used to be able to start composing an email in a new Thunderbird window, then switch back to my Inbox window. I could select a message (or multiple messages) and drag them down to my Compose Message item in the taskbar (without releasing my mouse button) to make the Compose Message window active. Then, while still holding my mouse button down, I could drag the still selected emails up to the message that I was composing and release my mouse button. This would add the messages as attachments. Since 11.04, I can no longer switch windows my dragging over the taskbar items, which makes it much more difficult to attach emails this way.