Topbar accessibility mobile and zoomed view

Describe the bug
The Deque audit of Teams raised three issues related to the mobile or zoomed view of the topbar:

1. Content is lost at 200% zoom
2. Content overlaps other content at 320px width
3. Horizontal scrolling at 320px

To Reproduce
Steps to reproduce the behavior:

1. Go to stackoverflow.com
2. Open the developer tools
3. Use the responsive emulator and select iPhone 12 (this will give you a similar experience to both a mobile user and a user operating the site at 200% zoom)
4. See the horizontal scrollbar that appears around the action menus and the lost information on the avatar

Expected behavior
The design of the topbar for smaller screens is adapted to better suit the available real estate as per the Figma file.

Functional specs

• Priority navigation for the action menu btns (see CSS Tricks or this example), which moves items into the overflow menu as the screen changes sizes.
• When outside the overflow menu, action menus are full width and use existing UI (see the search example)
• When inside the overflow menu, action menus open like an accordion and use existing UI (see the help example)
• The overflow menu can only have one open action menu at once
• Entire block for menu items (e.g. “Help” in the overflow menu) and subitems (e.g. the Help subitems) are clickable
• Sub menus and dropdowns inherit the theme (Teams is always dark, public depends on user’s theme)
• There must be at least a 96px gap (TBC; this should be configurable so we can play with it) between the end of the team name or SO logo and the first menu item unless the first possible item is the overflow menu, in which case the team names gets a ... at the end (rather than wrapping). Exception: logged out view.

Start by reproducing the topbar problems in the developer-tools responsive emulator using the iPhone 12 view, then compare the behavior with the linked Figma design and functional specifications. Done means no content loss at 200% zoom, overlap or horizontal scrolling at 320px, and the specified overflow-menu, theme, click-target, and spacing behavior is met.