New navigation dropdown does not read current collapsed state before opening

User bar "Merge requests" button does not have aria-expanded="false" by default, only announces "expanded"/"collapsed" after being toggled.

Solve for custom toggle elements of GlDisclosureDropdown in super sidebar to set aria-expanded="false" by default before button is pressed and aria-expanded attributes gets updated.

See findings and recordings from new navigation accessibility screen reader review &11510 (closed)