Cache element items on creation

[jarle]

Continuing from atom/atom-select-list/pull/21. The gist of it is: cetain performance issues in command-palette is caused by the fact that elementForItem is invoked for every item whenever the list state is updated, causing command-palette to create and recreate every DOM element more often than needed.

A simple solution to the problem, which is implemented in this PR, is to cache item elements on creation. Items are cached and added incrementally as the selection or query changes.

[nathansobo]

👏 👏 👏 Bravo @jarle. Thanks for your dedication on this issue and being patient with my incomplete feedback. Sometimes it's a process of discovery. Glad you took this up. Thanks so much.

[nathansobo]

Published as 0.42.1 and upgraded in atom/atom@377eeea.

[jarle]

Thanks, I have appreciated the feedback and suggestions a lot! Happy to see this merged.

[jarle]

So it seems that the element cache is not utilized at all between command-palette toggles. This seems to happen because the items array passed to the underlying selectList is regenerated every toggle, causing the old WeakMap item keys to be obsolete. A solution could be to switch the top-level WeakMap to a Map, where the underlying commandName would be used as key.

EDIT: After looking at this today, I think perhaps the garbage collection plays a significant role in making the WeakMap keys obsolete. The solution in #95 should should still be valid.