See https://wordpress.org/support/topic/weird-checkbox-behaviour-on-wp-7/

Sort of demo: https://www.youtube.com/watch?v=w-JWX1eudGE

Essentially some of our CSS is making it difficult to see current checked state while checkboxes are still focused, making it appear that clicking the checkbox has not done anything, until you move focus away and realize it did get checked.

Source:

.cmb2-wrap input:focus, .cmb2-wrap textarea:focus{ background: #fffff8;}

https://github.com/CMB2/CMB2/blob/develop/css/sass/partials/_main_wrap.scss#L85-L88

Overriding workaround includes:

.cmb2-wrap input:focus, .cmb2-wrap textarea:focus {
  background: inherit;
}

.cmb2-wrap input:checked, .cmb2-wrap textarea:checked {
  background: var(--wp-admin-theme-color);
}

However, the :checked version may not be needed if _main_wrap.scss#L85-L88 is removed as a whole.

If we want to provide initial back compat, we could prefix with .version-7-0 but I feel this is probably a small enough change we could just outright remove, especially going forward while people eventually upgrade everything.

cc @jtsternberg