When use question mark to open the keyboard help, then we expect to use the question mark to dismiss the keyboard help.
Does any one could help on this?
You could also try posting at https://meta.discourse.org/ in the "feature" or "bug" category.
The keyboard shortcuts panel can be closed with the
esc key, but in Safari it will also exit full screen mode.
Examples of toggle keys in Discourse are
= (hamburger) and
p (user profile), but those are for menus rather than panels.