This might have been up before but I can’t find it…
+1
This would be very helpful!! As a work-around in the meantime, you could make some changes on the page itself (which shows the content) without actually making changes, then go to your Master Page Editor and make your actual changes that you desire there. It’s definitely more fiddly, but may help. I agree that your request is really good to have this behavior right in the Master Page Editor.
EDIT - After reading this post I realize that it’s not the most clear thing I’ve ever written. Apologies… I meant you can make adjustments on the actual page but don’t save the changes, taking note of desired adjustments, then apply those in the Master Page Editor.