New Course Unit page: Improving the accessibility of the Сourse unit page

Possible targeted accessibility improvements for the course unit page:

• sidebar elements are focused only if you navigate from the bottom up (often users navigate from the top of the page, down)
• when switching navigation tabs, the focus is not saved on the active tab (the user needs to move to the navigation panel again in order to switch to the next one)
• if there are many units, the user is forced to go through each of the units to get to the active one
• aria attributes of the navigation panel are not accessible to users
• the anchor link "Skip content" does not work
• there is no focus on the UI side for the dropdown elements of the unit's xblocks
• the dropdown elements of the xblocks are visible to the screen reader if the dropdown is not displayed

Some references:

• https://www.ucdenver.edu/centers/center-for-inclusive-design-and-engineering/community-engagement/colorado-assistive-technology-act-program#ft-training-and-assistance-1

https://github.com/user-attachments/assets/29419cc9-ffae-4a34-ac4b-6dfa786be419

Start by reproducing the listed keyboard-navigation and screen-reader problems on the course unit page, focusing on the sidebar, navigation tabs, navigation panel, Skip content link, and xblock dropdowns. Inspect the page entry point and related components, then verify that focus order, active-tab focus, ARIA behavior, hidden dropdown visibility, and skip navigation work as described for assistive-technology users.