Several times this has happened! I need to correct a message I’ve been keying in on the Reply screen, and so I position the cursor in the specific place and start to click Backspace, etc., to remove or add a letter or space, or make a correction. I then see the hourglass cursor, or its equivalent, and the screen picture reverts to the last one I’d seen! Is this another “cookie” problem, or is it just me? 
Do you mean the back arrow? In some browsers you might be able to set up that key to bring you back to the previous page viewed in your browser. It’s most likely a browser setting.
Let me guess–you’ve got Windows, right?
It’s a Microsoft thing. Drove me CRAAAAAZY until I figured out what was going on. Microsoft has the Backspace key for “go back a page”, and there’s isn’t any way to change it. So here’s what you do.
You make dead certain that you’ve clicked the mouse in the Reply window, and have waited a beat or two until you see the cursor start blinking, so you know the cursor is in the Reply window, before you hit the Backspace key. Otherwise, if you don’t wait until you see the cursor blink in the Reply window, as soon as you hit the Backspace button, you’re going to see the previous screen.
And when that happens, just click on Forward with the mouse, up in the Top Left corner, under File and Edit, where it says <— Back V —>
That right-facing arrow is actually a “Forward”, and it ought to bring your nicely typed Reply window back again.
—Forward again.
Whatever.
Thanks to both; I see your point. 
This happened not only on the SDMB but on the Message Board for my alma mater, Redondo High in Redondo Beach, CA, on the Classmates website.