Flyout Panels
After a project is opened, triangular icons for flyouts panels appear to the right of the Activity Bar. Click a triangle to open the corresponding flyout panel. A second click causes the panel to close.
Additional information:
CoCalc feature announcement and discussion: ANN: Flyout Panels for Projects.
CoCalc News article introducing flyouts: Flyout Panels for Projects.
CoCalc News article with updates: Update on Flyouts.
Below are screenshots illustrating each flyout panel: