This facilitates access to the meta box pane through use of the mouse wheel or touchpad scrolling gestures. There are a couple of ways it’s engaged.

1. Automatically when scrolling to bounds
   When the post content is scrolled to the end it reveals the meta box pane. When the meta box pane is scrolled to the start it hides the meta box pane.

2. At will, in combination with keyboard input
   Press and hold the Control/Ctrl key and use the mouse wheel or touchpad scroll gesture over either the post content or the meta boxes to adjust the split while maintaining scroll positions in both the content and the meta boxes.

Both of the behaviors are enabled by default. If either one isn't helpful it can be disabled in the sidebar.