Slight tweaks to make things jump around less when clicking on a topic

Still jumping a bit, but less than before. This may be addressed later.

One issue is a problematic bootstrap rule for nav-stacked which matches in the actual topic tree, but not when that portion of the tree gets swapped out for the placeholder element. The rule is this one:

.nav-stacked > li + li {
  margin-top: 2px;
}