How to use blank key in sequences
How do use the sequence in the attached image? I have had no luck in determining this, and I know it is NOT the space key.

The character should be U+0338 “COMBINING LONG SOLIDUS OVERLAY” which is probably not present on any keyboard anyway. Some Xorg setups allow to type this sequence but I don’t think it can be done with WinCompose, so if you want to type in this symbol you should create your own additional sequence. I think I may just hide any sequences that cannot be typed in, to avoid confusion.
Hi! I’m happy to report that WinCompose 0.9.7 no longer renders the box as empty. This is only one half of the bug fix, now I need to make sure there is feedback for the user when a character cannot be typed with the current keyboard layout.