In bug 1779578, I changed the way a11y trees are serialised such that the parent id is included for each Accessible.
When de-serialising in DocAccessibleParent::RecvShowEvent, in order to avoid a theoretical performance regression caused by repeatedly looking up the same parent, I added an optimisation to use the last parent if it is the same as the current parent.
Unfortunately, it seems I never actually set the lastParent and lastParentID variables, so this optimisation was a no-op!
This is a micro-optimisation: it doesn't seem to make any observable difference.
However, it seems silly to have effectively dead code and it's a very straightforward fix.
Differential Revision: https://phabricator.services.mozilla.com/D213177
808f8aaefd