Design: Cmd + F should select all text in the new file viewer search box

[marcleblanc2]

Feature request description

In Chrome / Edge / Arc / Safari / Firefox:

• Cmd + F will always select all content in the browser's little search box so that the next character you type will replace all text in the search box, regardless of
  -- If the search box was already open or not, or
  -- If the cursor's focus was already in the search box or not

In Sourcegraph's new file viewer:

• Cmd + F will only select all content in the new little search box when
  -- Opening the search box
  -- Moving cursor focus back to the search box
• Cmd + F does not select all content in the new little search box when the cursor focus is already in the new little search box

Is your feature request related to a problem? If so, please describe.

• User velocity
• Distraction from non-standard behaviour

Describe alternatives you've considered.

Workarounds currently available:

• Use Cmd + A to select all text in the new little search box
• Close the new little search box by pressing Esc, then reopen it with Cmd + F
• Move the cursor focus out of the new little search box by pressing Tab or clicking outside of the box, then move the cursor focus back with Cmd + F

Additional context

Requested by customer 578 in support ticket 13564 / Slack thread.