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This is a vertically oriented hamburger which is also available as a part of the options in the left section of the Library.
This divider icon can be dragged to either right side or left side, it allows you to increase or decrease the space that each of the side options occupies, so that you could work conveniently on an area or to help you see more conveniently as it bolds the objects in the more spacious area of the screen.
See image reference below:
A screenshot showing the divider icon