N-Panels can be opened and closed by dragging the arrow from the left to the right. Bottom regions (Headers as in the Preferences UI and File Browser), however, cannot be resized once the mouse is released from the original drag which is inconsistent behavior.
I was told that this is because “Headers in Blender can’t be hidden by dragging their borders, so as to make easier to resize editor regions without accidentally hiding headers.”
Since the user interface is the same for a collapsible region and a non-collapsible header, it’s confusing and appears broken. Perhaps using a [+] instead of a [^] would indicate a different behavior? Or a change of the cursor to a Prohibited Sign (🛇) when the Header can’t be dragged closed?
- Edit > Preferences
- Drag the arrow at the bottom left up to reveal the Preferences Header
- Attempt to drag it closed like an N-Panel