For a while when first using the viewer I found that one of my gestures didn't work. Recently I messed with the keys.ini file to try and use space as a button but it did nothing. Trying a different key worked.
I downloaded the source code and after tracing the input code I found that on non-Windows systems the space key isn't translated correctly and thus the key is dropped from input.
The fix for this is as simple as inserting the following in these files:
As an important side note: I don't use Mac so I googled the key code which may be wrong.