accessibility - If spacebar opens dropdowns across all browsers, why is my onchange triggered menu considered inaccessible -

background: windows users on chrome , ie, dropdowns reload or change page no accessibility. user presses down arrow button, page reloads. means user can access first menu option. here example: http://html.cita.illinois.edu/script/onchange/onchange-example.php

this covered in wcag rule: “changing setting of user interface component not automatically cause change of context unless user has been advised of behavior before using component. (level a)”

except user can open dropdown , explore options without triggering onchange event. user space bar press. commonly known keyboard trick i've seen tested users aware of or able figure out quickly.

in system, using dropdown pagination in long directories. eg: "you on page [1^] of 16" (with [1^] being browser default dropdown menu). designers not allow kind of visual [go] button. happens across thousands of pages, javascript fixes i've seen need account every dropdown, , impossible on our case.

using space bar, user can see options , make selection anywhere on list using keyboard. why dropdown page menus automatically reload onchange still considered inaccessible? , considered accessible if included screen-reader text said "press space bar open menu, making selection load next page"

well except in firefox, keyboard command f4, correct - not wcag 2 violation rather best practice

here page can test this: http://dylanb.github.io/onchange_select.html