Two regions, one negotiable boundary
The sidebar and the canvas, the list and the detail, the editor and the preview. The split is a percentage, so it survives container resizes, and panes clip their content rather than growing the page. The divider is a real separator: focusable, draggable with pointer capture, and nudged by arrow keys.
Horizontal
Drag the divider, or focus it and use the arrow keys. Hold Shift for bigger steps; Home and End jump to the limits.
Vertical
The same component stacked. The separator reports its orientation and the arrow keys follow it: up and down instead of left and right.
Bounded sidebar
minSplit and maxSplit fence the drag, so a sidebar can be squeezed but never collapsed away. This one starts at a quarter and moves between 15 and 40 percent.
Scrolling regions
A region that should scroll brings its own focusable scroll container inside the pane, so keyboard users can reach it. The long list here scrolls against a fixed neighbour instead of stretching the layout.