New change to Twitter, looks like they are moving to make it easier to communicate
When posting my co-worker’s blog on Swacket late Tuesday night I discovered that when replying to tweets in the mentions view, the box has changed.
When you reply, the entry box is now under the original tweet, removing the popup. Looks like, just as with the recent announcement of expanded tweets, Twitter is moving toward a more streamlined approach to encouraging conversation on the platform. 
All the better for everyone I say. Easier navigation is always a delight to users.