Design idea for [link]

Multicolumn when Hovering

Immediately when clicking Places, show a number of bookmarks set by the user (without needing to use scrolling or other conventions), then when the user highlights any of the 7 visible bookmarks, fade-in an extension to the list immediately to the right of the menu, with a size that is either an arbitrary height or the same height/count as the number of originally visible bookmarks.

after that, 2 options:
1) continue that pattern as the user moves to the right, continuously expanding this matrix to fill the extended list, or
2) load all of the matrix at once, showing the user all their bookmarks when highlighting any of the original 7.

I suggest showing an actual grouping box for the original bookmarks area with a different color than the rest of the items in the Places Menu.

This might be complicated to code, but it's the most intuitive and easiest to use repeatedly.