Is it acceptable to spell it “spacebar” or must the word be spelled with a space in it? I’m using it to refer to a keyboard shortcut in my application’s documentation.
Further, should it be capitalized? And if split into two words, should both be capitalized?
Definitely not capitalized: a Space Bar would be something like a Mars Bar or Milky Way, but less dense (facetious confectionery reference).
But whether you include a space is up to you. Originally so: the space bar is just below the X key, which requires a space to avoid confusion. However, spacebar is common enough not to be ambiguous by now.