Drag or arrow the divider
Two panes with a movable divider. Drag it, or focus it and use the arrow keys; Home and End go to the bounds. The size is resolved in the runtime and written onto the container as a custom property, because a pointermove fires far too often to route through a client function.
<LayoutSplitter
size="40"
minSize="20">
<div data-slot="start">
<div class="showcase-slot">start slot</div>
</div>
<div data-slot="end">
<div class="showcase-slot">end slot</div>
</div>
</LayoutSplitter>