Mobile File Tree Browser: click outside the component closes only when applicable

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Context

when the file tree browser panel is open, and users click into the page area outside the file tree browser, the panel should automatically close. This is a general web pattern that gives users faster access to the element they are trying to focus/click on (if they're trying to focus outside of the file tree browser panel). See #569507 (comment 2851694452)

Design proposal

See #581018 (comment 2958387035).

• In moible viewport sizes where the file tree browser panel appears, add a behavior that allows users to click outside the component to close the file tree browser.
• This behavior only applies to mobile viewport sizes < 768px.

For reference, the viewport breakpoints in #583851 are:

Viewport	Proposed
xs <576px	Mobile. Panel. To close the panel, click on the file tree button or click anywhere outside the panel #581018
sm ≥576px	Mobile. Panel. To close the panel, click on the file tree button or click anywhere outside the panel #581018
md ≥768px	Side-by-side
lg ≥992px	Side-by-side
xl ≥1200px	Desktop. Side-by-side.