Dependent clauses functioning as subjects are treated as singular:

(1) [That Paul might be a thief] has never occurred to Mary.

(2) [What he failed to understand] was how she managed to escape.

If you were to argue that it is actually how she managed to escape which is the subject, you may be right, but it would not change the fact that clauses take singular agreement, since how she managed to escape is also a clause.

When two dependent clauses, for instance two non-finite dependent clauses with present participles as predicate verbs, are conjoined and together constitute the subject, we get plural agreement, as in the following example:

(3) [[Listening to music] and [watching movies]] are my favourite pastimes.