No idea on the ALT+Enter issue... if I *don't* have my focus set to the editor, then ALT+Enter appears to be going full screen on me
Yea if your focus is on the page, but if your focus is on the search field... this is the scenario:
Hit reply, type in the editor field
Goto the browsers search field, type something and hit alt + enter (to open the results in a new tab)
Now click off the editor field
Goto the browsers search field, type something and hit alt + enter
New tab opens with results
(the old editor did this as well, it might be related to the browsers native wysiwyg interface).