fix about:reader in system.css after fixing computer about uc.css.js HOT 2 CLOSED

Also fix tabbrowser-arrowscrollbox's scroll buttons. They need to fully collapse on [scrolledtostart][scrolledtoend] instead of just [overflowing] since the rounded outside corners cause the scrollbox to receive overflow events when it's not actually overflowing. e.g. the tabs are still flexing but it gets attribute overflowing just because the corners overflow by 5px. That causes the scrollbuttons to be visible when they shouldn't be.

Also, something needs to be done to truly collapse scrollbutton bounds when not drag/dropping, since the closing spacer grows when closing tabs with the mouse, while the scrollbox isn't overflowing. That causes like 16px of empty space next to the final tab, until mouse leaves the navbar.

from uc.css.js.

Fix by ddd203e

from uc.css.js.