Just tried it out. The space bar is certainly convenient to reach and in practice, adding a space to the Search Edit Box is not a hardship, since spaces are ignored in Instant Search. It's only the look of it, that there are spaces between the LH end of the field and any added text. If you fill the Search Edit Box with spaces so that the text cursor is to the far right of the field, adding any further text will keep the cursor at the right hand end and the added text expands leftwards. , which is a bit unusual for PB! In other words, if you move the mouse cursor over a Thought (capable of expansion) in Outline View and press the spacebar with the non-mouse hand, This feature works only with keyboard navigation nothing happens. (I hope they make that clear in the User Guide when they get round to updating it.) It only works if you select the Thought by keyboard navigation (move the green ring onto it), then press the spacebar. But I agree, I can see no reason why we can't use some other accelerator instead of the spacebar here. Maybe a UserVoice request would assist them to reconsider this aspect. Another amusing thing about this feature is that it is also activated by typing spaces search terms. E.g., if you type " within three blind mice" and the green ring happens to be over an expandable Thought, Outline View expands and collapses the Thought before your eyes, as you type into the Search Edit Box! This would not impress customers during a presentation.
PB 18.104.22.168 on Windows XP, J-1.6.0_17