fix scroll issue with checkbox plugin when query has content

[formigarafa]

This is to fix a scroll issue with checkbox_options plugin.

Steps to reproduce the issue:

1 - have a multi-select with checkbox_options plugin enabled with enough options that it is still scrollable with something typed in the search box. 2 - open the select, type a query with partial match that matches enough options to allow scrolling. 3 - scroll to bottom of the list 4 - mark 2 checkboxes

problem: after selecting the second option the list scrolls to the top making it cumbersome to select near options.

expected behaviour (fixed with this PR): Checkbox to be marked and selection to stay visible so we can continue selecting further records.

[ericscales]

@nwalters512 what's needed to get this one merged?