add some comments

ui/app/mixins/with-nav-to-nearest-ancestor.js

@@ -2,6 +2,15 @@ import Mixin from '@ember/object/mixin';
 import utils from 'vault/lib/key-utils';
 import { task } from 'ember-concurrency';
 
+// This mixin is currently used in a controller and a component, but we
+// don't see cancellation of the task as the while loop runs in either
+
+// Controller in Ember are singletons so there's no cancellation there
+// during the loop. For components, it might be expected that the task would
+// be cancelled when we transitioned to a new route and a rerender occured, but this is not
+// the case since we are catching the error. Since Ember's route transitions are lazy
+// and we're catching any 404s, the loop continues until the transtion succeeds, or exhausts
+// the ancestors array and transitions to the root
 export default Mixin.create({
   navToNearestAncestor: task(function*(key) {
     let ancestors = utils.ancestorKeysForKey(key);
@@ -13,6 +22,8 @@ export default Mixin.create({
         transition.data.isDeletion = true;
         yield transition.promise;
       } catch (e) {
+        // in the route error event handler, we're only throwing when it's a 404,
+        // other errors will be in the route and will not be caught, so the task will complete
         errored = true;
         nearest = ancestors.pop();
       } finally {