We have multiple users, on different operating systems and different browsers, that are getting the same issue.
When they hit 'reply all' in the Preview Pane on the right-hand side, the resulting window to respond, the send, discard and insert buttons are not visible. They are pushed up under the blue Office 365 'logo bar' on the top of the page.
The only way I have had success in fixing this is to have them go to the zoom level in their browser and zoom out. Another option is to have them display the reading pane to the bottom but this is not preferred.
What change occurred that is forcing the inbox page rendering to push these mail buttons so high on the page?
The affected OSes: Mac OS X, Windows XP and Windows 7
Browsers affected: IE 9, Firefox, Safari