Keep "peek" sidebar open while mouse hovers flyout menu

What does this MR do and why?

Fixes #419769 (closed)

Screenshots or screen recordings

Before	After
recording_1690373355	recording_1690373203

How to set up and validate locally

• These flyout menus were initially not part of the navigation redesign. They are currently still behind their own feature flag, so in your gdk rails console, run:

Feature.enable(:super_sidebar_flyout_menus)

• Then, make sure you have the new navigation toggled on in your user dropdown.
• Browse to any project or group, or the admin page, because their sidebars all have a lot of sections with flyouts.
• Now make sure your sidebar is closed.
• Move your mouse cursor to the left edge of the window to make the sidebar go into "peek" mode.
• Move your mouse cursor over a section title (like "Plan", "Settings", etc.) to see its flyout menu.
• Hover over to the flyout menu.
  • On master, the flyout menu will disappear together with the sidebar, because you moved your cursor away from the sidebar.
  • In this branch, the flyout menu and sidebar will stay open while the cursor is in either of the two.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.