Both advertised behaviour they did not have. LayoutSplitter declared resizeStart, resize and resizeEnd with no pointer handling whatsoever, so a caller wired up @resize and received nothing, for ever, with no error, and its props were columns, gap and maxWidth copied from a grid scaffold. CustomScrollbar was the same shape with a scroll output. The splitter now resizes. Dragging lives in the reactive runtime behind data-wrn-splitter, because a pointermove fires far too often to route through a client function and a state write made in that callback is dropped; the resolved size is held on the container as a --wrn-split custom property and the component grids from it. The handle is a real separator: arrow keys step it, Home and End go to the bounds rather than to nothing, and it carries aria-valuenow, aria-valuemin and aria-valuemax. minSize fixes both bounds so neither pane can be dragged away and left unrecoverable. CustomScrollbar is CSS rather than script -- scrollbar-width and scrollbar-color with webkit rules for the engines that still need them -- and its fake scroll output is removed rather than left unimplemented, since a caller can listen for a plain scroll event. The test harness needed a fix too: mount did not bind the window CustomEvent, so the runtime built events from the host global and happy-dom listeners never matched them, which made anything dispatched look silently lost. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>