We've just upgraded to v1.8.1 and the users on my forum are experiencing numerous problems. One of them seems to happen on numerous browsers and operating systems.
I have filed a detailed bug report on GitHub, but the issue boils down to this: the position of the toolbar and/or editor when hitting reply is dependent on the length of the thread. If you reply to the first post, things will work OK:

If you reply to the second post, you'll note that the editor window is now starting much higher on the page:

Finally, if you reply to the last post on a long thread you'll see the following and the editor will be unusable. (Or at least seem to be... when you type, you see nothing, but maybe the text is so far above the viewable area you can't see it.)

As you can see in the user discussion here:
This is affecting a number of users on numerous browsers. Clearing the cache does not improve the issue.
Best Wishes,
Mark