[Meta] Pre-flight bug cleanup for rolling out Repository File Tree Browser

Context

This issue aims to reserve capacity during 18.7 to fix last-minute bugs as identified by our testing.

It also aims at simplifying tracking these for clear status representation.

Covered tasks/bugs

Blocker for internal release

• @jerasmus Search entire repository instead of client-side... (#543753 - closed)
• @jerasmus Internal release: Add UI to capture feedback (#581260 - closed)
• @JoFletcher Keyboard navigation and focus management for fi... (#561563 - closed) (!212533 (merged))
  • Restoring focus to the toggle call-to-action
• @jerasmus Improve file tree browser/panelled UI sticky pa... (#579330 - closed) (!214023 (merged))
• @jerasmus Encoding issues when clicking or ref/dir/files containing special chars (!213677 (merged)).
• @jerasmus We need to preserve ref_type in the URL while navigating the FTB (!213859 (merged))

Blocker for end user release

• @ms.mondrian [BUG] File tree browser mode does not update wh... (#569507 - closed) (see this comment)
  • Close button showing when it shouldn't
  • xl viewports ≥1200px, the file tree browser is supposed to appear side-by-side with the content.
• @jerasmus Repository File Tree Browser: unwanted horizont... (#581016 - closed)
• @jerasmus #567775 (comment 2851589520) In the old UI, the top of the file tree browser is overlapping the separating line below the primary breadcrumbs. There should not be an overlap (!214033 (merged))

Nice to haves:

• @jerasmus When Repository File Tree Browser is closed it should not be accessible to user navigation (!214186 (closed))
• @ms.mondrian Update the selected element to have a border-radius of 8px. #567775 (comment 2851589520) (!214276 (merged))
• @ms.mondrian Add hover color to a selectable line item to design token button.default.tertiary.background.color.hover (light gray), and change the color of the selected element to button.confirm.tertiary.background.color.hover (light blue). #567775 (comment 2851589520) (!214276 (merged))
• @jerasmus In the lg viewport, when the panel is open, the X close button is redundant with the file tree browser button. Remove the X. #569507 (comment 2898020781)
• #569507 (comment 2851692695)
• Click outside FTB to close peek mode (mobile)

Known issues without priorities assigned

• Auto-scroll to current dir/file in the reposito... (#541152)