On the timeline screen the arrow to hide the sidepanel when working on an event or era is hidden by the top panel with the notebook, news ect. Only when I changed the browser zoom to 75% was I able to see the hide arrow on the top right of the side panel.

On 1920x1080 resolution
Using OperaGX
Open Timelines
Create an event, it should open the sidepanel to edit it further and add descriptions

The < arrow to close the panel is hidden behind the top middle panel at 100% zoom