Let's get this out of the way first: This is a totally self-serving post.
FBReader, an e-book reader that can handle EPUB, has a feature to go full-screen. This is very useful--but also quite confusing. None of the menubar buttons seem to enable/disable this feature. Yet, it's fairly easy to accidentally turn on or off--and completely baffling when it happens. A quick google turned up nothing really obvious about how to enable/disable/toggle fullscreen mode. Thus, this blog entry.
Without further ado, here's the shortcut key: <RET>. Yes, the return key or enter key is the way you toggle fullscreen, full screen, full-screen off and on. And this does make some sense. The return key is pervasive on most PDAs, handhelds, and certainly on the author's laptops. So, it is nearly universally available (and moreover the enter key makes little sense in the body of a more-or-less read-only document format like EPUB.)
I've intentionally loaded this blog entry with searchable terms so that others trying to figure this out can easily find the information. That's why keyboard shortcuts and the word fullscreen appears several times with various spellings. Likewise, FBReader appears repeatedly. FBReader, which has nothing to do with FaceBook and does not predate FB, was certainly something I was aware of prior to my knowledge of facebook.
I don't yet own a Kindle, Nook, Sony Reader or any other dedicated ebook reader. Moreover, I don't have an iPhone, Android phone, Palm or other smartphone. I do have a Nokia that can apparently also run FBReader and I may try that out at some point but I have not yet done so. And no, I've never owned the original FBReader device, Sharp Zaurus.