Describe the bug
When jumping through the main ARIA landmarks in the editor (also known as "navigate-region" component) via the dedicated keyboard shortcuts, the focus style of the regions is broken, in slightly different ways on different browsers.

To reproduce

• Test in different browsers, also on Windows.
• Use the keyboard shortcuts to navigate through the landmarks.
• on macOS:
  • Ctrl+backtick
  • Shift+Ctrl+backtick
• on Windows (if I'm not mistaken):
  • Shift+Alt+N
  • Shift+Alt+P
• see the focus style is broken on some of the regions

Expected behavior
The focus style to always be clearly visible.

Note
This style broke a few times during the project history. Maybe worth considering a different, more solid, testable, implementation.

Screenshots

With Gutenberg plugin activated.

Chrome Windows: content region focused: the style is visible only in the part of the UI close to the browser scrollbar:

Chrome Windows: sidebar region focused: the style is barely visible:

Chrome macOS content region: no style visible:

Chrome macOS sidebar region: style barely visible:

Chrome macOS publish region: no style visible:

Firefox macOS sidebar region: style breaks but in a different way:

Editor version (please complete the following information):

• can reproduce with slightly different results on both:
  • default editor shipped with WP 5.4
  • plugin version 7.9.1