Selectmenu: Correctly update checkboxes in non-native multiselect list

(cherry picked from commit dca863f493502ec578fa86e5a7be7e0e24d0e150)

Closes gh-7774
Fixes gh-7732
Selectmenu: Fix up coding style

(cherry picked from commit 8790743f95818cd344d0bab2179eb923c8c045d9)

Closes gh-7806