Nav unification - allow sidebar to be scrolled (while maintaining flyout menus)

[frontdevde]

Note: This is a separate issue for a task that was previously part of #46094.

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We recently merged the first iteration of flyout menus in #46046 (comment). One of the known issues, that we'll have to address next:

Scrolling inside the scrollbar vs scrolling the sidebar with the page

Right now in Calypso the sidebar container is position:fixed and it's overflow content (i.e. more nav items than screen height) is scrollable. This is achieved by adding overflow:auto to the sidebar. The issue is that overflow:auto prevents overflow content i.e. the flyout from being visible.

We removed the overflow:auto on the sidebar for flyout menus to be visible in the nav unification prototype. This has two side effects: a) the sidebar container isn't scrollable anymore and b) when using the site switcher, the overflow is visible when selecting a site.

wp-admin gets around the first issue by not having a scrollable sidebar container but instead adjusting the sidebar positioning via JS based on scroll state. The second issue is not present in wp-admin as there's no in-page site switcher.

[cpapazoglou]

After merging #46689 we just need to make the transition from the site-switcher back to the menu smoother by hiding the overflowing menu till transition is over

This is what is happening currently:

[getdave]

After merging #46689 we just need to make the transition from the site-switcher back to the menu smoother by hiding the overflowing menu till transition is over

Should we make a dedicated Issue for that so it's easier to track what is left to complete? Or does it fit nicely within this Issue?

[cpapazoglou]

Done Transition from the site-switcher back to the menu animation. Updated the master issue too.