feat(ui): build LayoutSplitter and CustomScrollbar for real

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>

packages/ui/components/CustomScrollbar.wrn

@@ -1,17 +1,122 @@
+// CustomScrollbar -- a scrolling region with a themed scrollbar.
+//
+//   <CustomScrollbar maxHeight="20rem">...long content...</CustomScrollbar>
+//
+// Scrollbar appearance is CSS, not script: scrollbar-width and scrollbar-color
+// are the standard properties, and the ::-webkit-scrollbar rules cover the
+// browsers that still need them.
+//
+// This component used to declare a scroll output it never emitted, and its
+// props were columns, gap and maxWidth copied from a grid scaffold. The output
+// is removed rather than left unimplemented -- a caller can listen for a plain
+// scroll event on the element, which is what it would have been anyway.
+//
+// NOTE: the style block uses /* */ comments only -- // is not a CSS comment
+// and silently swallows the rule that follows it.
 component CustomScrollbar {
-  outputs {
-    scroll(payload: { sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])
-  }
-
   props {
-size: string = "default"
     color: string = "primary"
-    columns: number = 2
-    gap: string = "md"
-    maxWidth: string = "xl"
+    size: string = "default"
+    axis: string = "vertical"
+    // Track thickness in pixels. Clamped, because it arrives as an attribute
+    // and a scrollbar wider than the content is not useful to anyone.
+    thickness: number = 8
+    maxHeight: string = "20rem"
+    radius: string = "999px"
     class: string = ""
   }
+
+  functions {
+    shared function trackSize() {
+      var value = Number(thickness)
+      if (!value || value < 2) {
+        return 8
+      }
+      return Math.min(24, value)
+    }
+  }
+
   view {
-    <div class="wire-next wire-next--color-{color} wire-next--size-{size} wire-next--custom-scrollbar wire-next--gap-{gap} wire-next--columns-{columns} wire-next--max-{maxWidth} {class}"><slot /></div>
+    <div
+      {...attrs}
+      data-ui-component="CustomScrollbar"
+      class='wire-scrollbar {class}'
+      data-color='{color}'
+      data-size='{size}'
+      data-axis='{axis}'
+      style='--scrollbar-thickness:{trackSize()}px;--scrollbar-radius:{radius};max-height:{maxHeight};'
+    >
+      <slot />
+    </div>
+  }
+
+  style {
+    .wire-scrollbar {
+      --scrollbar-accent: var(--wire-color-primary);
+      min-width: 0;
+      overflow: auto;
+      /*
+       * The standard properties. Firefox and current Chrome honour these; the
+       * webkit rules below are only for the engines that still ignore them.
+       */
+      scrollbar-width: thin;
+      scrollbar-color: color-mix(in srgb, var(--scrollbar-accent) 55%, transparent) transparent;
+      overscroll-behavior: contain;
+    }
+
+    .wire-scrollbar[data-color="secondary"] {
+      --scrollbar-accent: var(--wire-color-secondary);
+    }
+
+    .wire-scrollbar[data-color="success"] {
+      --scrollbar-accent: var(--wire-color-success);
+    }
+
+    .wire-scrollbar[data-color="danger"] {
+      --scrollbar-accent: var(--wire-color-danger);
+    }
+
+    .wire-scrollbar[data-color="info"] {
+      --scrollbar-accent: var(--wire-color-info);
+    }
+
+    .wire-scrollbar[data-axis="vertical"] {
+      overflow-x: hidden;
+      overflow-y: auto;
+    }
+
+    .wire-scrollbar[data-axis="horizontal"] {
+      overflow-x: auto;
+      overflow-y: hidden;
+    }
+
+    .wire-scrollbar::-webkit-scrollbar {
+      width: var(--scrollbar-thickness, 8px);
+      height: var(--scrollbar-thickness, 8px);
+    }
+
+    .wire-scrollbar::-webkit-scrollbar-track {
+      background: var(--wire-color-surface-soft);
+      border-radius: var(--scrollbar-radius, 999px);
+    }
+
+    .wire-scrollbar::-webkit-scrollbar-thumb {
+      background: color-mix(in srgb, var(--scrollbar-accent) 45%, transparent);
+      border-radius: var(--scrollbar-radius, 999px);
+    }
+
+    .wire-scrollbar::-webkit-scrollbar-thumb:hover {
+      background: var(--scrollbar-accent);
+    }
+
+    /*
+     * A pointer-less device paints its own overlay scrollbar and ignores the
+     * width above, so the region keeps its own padding instead.
+     */
+    @media (hover: none) {
+      .wire-scrollbar {
+        scrollbar-width: auto;
+      }
+    }
   }
 }

packages/ui/components/LayoutSplitter.wrn

@@ -1,19 +1,210 @@
+// LayoutSplitter -- two panes with a divider the reader can move.
+//
+//   <LayoutSplitter size={40} minSize={20}>
+//     <div data-slot="start">...</div>
+//     <div data-slot="end">...</div>
+//   </LayoutSplitter>
+//
+// The dragging lives in the reactive runtime behind data-wrn-splitter. Pointer
+// moves fire far too often to route through a client function, and a state
+// write made inside a pointermove callback is dropped, so the resolved size is
+// held in the DOM as the --wrn-split custom property and these styles read it.
+//
+// This component previously declared resizeStart, resize and resizeEnd with no
+// pointer handling whatsoever: a caller wired up @resize and received nothing,
+// for ever, with no error.
+//
+// NOTE: the style block uses /* */ comments only -- // is not a CSS comment
+// and silently swallows the rule that follows it.
 component LayoutSplitter {
   outputs {
-    resizeStart(payload: { sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])
-    resize(payload: { sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])
-    resizeEnd(payload: { sourceEvent?: Event; value: string | number | boolean | null | object; [key: string]: string | number | boolean | null | object } | string | number | boolean | null | object[])
+    resize(payload: { size: number })
   }
 
   props {
-size: string = "default"
     color: string = "primary"
-    columns: number = 2
-    gap: string = "md"
-    maxWidth: string = "xl"
+    size: number = 50
+    orientation: string = "horizontal"
+    // Smallest share either pane may take, as a percentage. It also fixes the
+    // upper bound at 100 - minSize, so neither pane can be dragged away to
+    // nothing and left unrecoverable by pointer.
+    minSize: number = 15
+    step: number = 5
+    label: string = "Resize panels"
     class: string = ""
   }
+
+  functions {
+    shared function isVertical() {
+      return orientation === "vertical"
+    }
+
+    shared function lowerBound() {
+      var value = Number(minSize)
+      if (!value || value < 0) {
+        return 15
+      }
+      return Math.min(45, value)
+    }
+
+    shared function upperBound() {
+      return 100 - lowerBound()
+    }
+
+    // Sizes arrive as HTML attributes, so a value outside the bounds is
+    // routine rather than exceptional. Clamp instead of rendering something
+    // the reader cannot undo.
+    shared function currentSize() {
+      var value = Number(size)
+      if (!value || value < 0) {
+        return 50
+      }
+      return Math.min(upperBound(), Math.max(lowerBound(), value))
+    }
+
+    shared function stepSize() {
+      var value = Number(step)
+      return value && value > 0 ? value : 5
+    }
+  }
+
   view {
-    <div class="wire-next wire-next--color-{color} wire-next--size-{size} wire-next--layout-splitter wire-next--gap-{gap} wire-next--columns-{columns} wire-next--max-{maxWidth} {class}"><slot /></div>
+    <div
+      {...attrs}
+      data-ui-component="LayoutSplitter"
+      class='wire-splitter {class}'
+      data-color='{color}'
+      data-orientation='{orientation}'
+      data-wrn-splitter='{isVertical() ? "vertical" : "horizontal"}'
+      data-wrn-splitter-min='{lowerBound()}'
+      data-wrn-splitter-step='{stepSize()}'
+      style='--wrn-split:{currentSize()}%;'
+    >
+      <div class="wire-splitter__pane wire-splitter__pane--start">
+        <slot name="start"></slot>
+      </div>
+
+      <div
+        class="wire-splitter__handle"
+        data-wrn-splitter-handle="true"
+        role="separator"
+        tabindex="0"
+        aria-label='{label}'
+        aria-orientation='{isVertical() ? "horizontal" : "vertical"}'
+        aria-valuenow='{currentSize()}'
+        aria-valuemin='{lowerBound()}'
+        aria-valuemax='{upperBound()}'
+      >
+        <span class="wire-splitter__grip" aria-hidden="true"></span>
+      </div>
+
+      <div class="wire-splitter__pane wire-splitter__pane--end">
+        <slot name="end"></slot>
+      </div>
+
+      <slot />
+    </div>
+  }
+
+  style {
+    .wire-splitter {
+      --splitter-accent: var(--wire-color-primary);
+      display: grid;
+      /* The first track follows the size the runtime resolves. */
+      grid-template-columns: var(--wrn-split, 50%) auto minmax(0, 1fr);
+      align-items: stretch;
+      width: 100%;
+      min-width: 0;
+    }
+
+    .wire-splitter[data-color="secondary"] {
+      --splitter-accent: var(--wire-color-secondary);
+    }
+
+    .wire-splitter[data-color="success"] {
+      --splitter-accent: var(--wire-color-success);
+    }
+
+    .wire-splitter[data-color="danger"] {
+      --splitter-accent: var(--wire-color-danger);
+    }
+
+    .wire-splitter[data-color="info"] {
+      --splitter-accent: var(--wire-color-info);
+    }
+
+    .wire-splitter[data-orientation="vertical"] {
+      grid-template-columns: minmax(0, 1fr);
+      grid-template-rows: var(--wrn-split, 50%) auto minmax(0, 1fr);
+    }
+
+    .wire-splitter__pane {
+      min-width: 0;
+      min-height: 0;
+      overflow: auto;
+    }
+
+    .wire-splitter__handle {
+      display: flex;
+      align-items: center;
+      justify-content: center;
+      flex: 0 0 auto;
+      padding: 0 0.25rem;
+      border: 0;
+      background: transparent;
+      cursor: col-resize;
+      /* Without this the pointer drag selects the text in both panes. */
+      touch-action: none;
+      user-select: none;
+    }
+
+    .wire-splitter[data-orientation="vertical"] .wire-splitter__handle {
+      padding: 0.25rem 0;
+      cursor: row-resize;
+    }
+
+    .wire-splitter__grip {
+      display: block;
+      width: 2px;
+      height: 100%;
+      min-height: 1.5rem;
+      border-radius: 999px;
+      background: var(--wire-color-border);
+      transition: background 140ms ease;
+    }
+
+    .wire-splitter[data-orientation="vertical"] .wire-splitter__grip {
+      width: 100%;
+      min-width: 1.5rem;
+      height: 2px;
+    }
+
+    .wire-splitter__handle:hover .wire-splitter__grip,
+    .wire-splitter[data-wrn-splitter-dragging="true"] .wire-splitter__grip {
+      background: var(--splitter-accent);
+    }
+
+    .wire-splitter__handle:focus-visible {
+      outline: 2px solid var(--splitter-accent);
+      outline-offset: -2px;
+      border-radius: var(--wire-radius-sm);
+    }
+
+    /*
+     * Two panes side by side stop making sense on a phone. Stacking them keeps
+     * both readable, and the divider stops being draggable because the grid
+     * no longer has a second track to trade against.
+     */
+    @media (max-width: 639px) {
+      .wire-splitter,
+      .wire-splitter[data-orientation="vertical"] {
+        grid-template-columns: minmax(0, 1fr);
+        grid-template-rows: auto auto auto;
+      }
+
+      .wire-splitter__handle {
+        cursor: default;
+      }
+    }
   }
 }