[Performance] Use “display: none” to hide options filtered by search

[conor-cafferkey-sociomantic]

Currently

position: absolute; left: -999999px; is used to hide dropdown options filtered by search.

Problem

With large datasets, moving the items off screen is slow. Setting display: none is much quicker when hiding the items and performs about the same when re-adding them^*.

Wanted

.flounder--search--hidden should set display: none.

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^*tested in Chome, to be verified in other browsers