Scroll to the bottom of the thread in many (though not all) forums and you will now see a Quick Reply button just below the last post. Clicking on this opens up a Post Reply Box at the foot of the page you are browsing. Type in your reply, hit Submit and you’re done -- no need to switch to a new screen via the Post Reply button. With Quick Reply, you can scroll easily between the thread you're browsing and the reply you're writing.
The downside: Quick Reply doesn’t give you access to the Colour Palette or any of the usual BBCode buttons (Quote, Bold, URL etc.). If you want to use those, stick with the ordinary Post Reply. (Or you can still type in BBcodes manually).
If you started typing in Quick Reply and want to switch to the full Post Reply screen, just click on the Full Editor button -- you can finish writing your post there. Note that there is no preview feature in Quick Reply.
This feature is not available in all forums, only those where replies tend to be short and fast. On serious discussion threads, we prefer users to take the time to phrase their response carefully and made use of the preview function! It really can’t be said often enough -- when dealing with sensitive and complicated subjects, no one should ever post from the hip. The Preview Button Is Your Friend...!!!