fix(layout): draw diagnostic tile markers inset + frame the sidebar viewport, not its scrolled content

Two renderer bugs made the diagnostic layout plugin's borders useless:

1. Borders went past the panels and were invisible at screen edges: CSS
   outlines draw OUTSIDE the border box by default, and tiles span their
   containers edge-to-edge, so the 4px ring overflowed the visible area
   and was clipped. The outline is now inset (outline-offset: -4px) so
   it draws inside the tile's box.

2. The green side border scrolled with the sidebar: the marker was drawn
   on the TilePanels root, which is the SCROLLED CONTENT of the split's
   collapsible child. The marker now lives on the split's scroll-viewport
   div — the sidebar's fixed, opaquely-painted frame: its box never moves
   (only its contents scroll), and its own outline is never clipped by its
   overflow. (The aside behind it can't carry the marker — the scroll
   viewport's bg-base-100 paints over an outline drawn on the aside.)

markerStyle() centralizes the full declaration (outline + inset offset)
and returns undefined for unmarked/unknown markers, so unmarked trees
still render with zero outline CSS (pinned by the existing tests).

Paint layering verified with a headless-Chromium screenshot of a probe
page reproducing the exact DOM + computed styles.

1 files changed, 35 insertions, 5 deletions

diff --git a/src/features/layout/ui/TileSplit.svelte b/src/features/layout/ui/TileSplit.svelte
index b41f9fc..b526ccd 100644
--- a/src/features/layout/ui/TileSplit.svelte
+++ b/src/features/layout/ui/TileSplit.svelte
@@ -6,9 +6,9 @@
    * in-flow push vs `max-lg:absolute` overlay with a backdrop. Collapse state
    * is chrome (per-node-id), owned by the injected collapse store.
    */
-  import type { LayoutOp, Path, SplitNode } from "@dispatch/layout-contract";
+  import type { LayoutOp, Path, SplitNode, TileMarker } from "@dispatch/layout-contract";
   import type { CollapseStore } from "../logic/collapse";
-  import { markerOutline } from "../logic/marker";
+  import { markerStyle } from "../logic/marker";
   import TileNode from "./TileNode.svelte";
 
   let {
@@ -36,10 +36,31 @@
     const key = keyOf(i, id);
     return state.openByNode[key] ?? state.defaultOpen;
   }
+
+  /**
+   * The scroll container's inline style: the fixed sidebar width + slide
+   * transform + any diagnostic marker (inset) — none when unmarked.
+   *
+   * The marker goes on THIS div, not the aside: the inner div paints an
+   * OPAQUE full-height background (bg-base-100) that would cover an outline
+   * drawn on the aside. This div's own box never moves — only its contents
+   * scroll — and overflow-y-auto clips descendants, never an element's own
+   * outline, so the marker frames exactly the visible sidebar.
+   */
+  function scrollViewportStyle(
+    basis: number | undefined,
+    open: boolean,
+    marker: TileMarker | undefined,
+  ): string {
+    const parts = [`width: ${basis ?? 320}px`, `transform: translateX(${open ? "0" : "100%"})`];
+    const markerCss = markerStyle(marker);
+    if (markerCss !== undefined) parts.push(markerCss);
+    return parts.join("; ");
+  }
 </script>
 
 {#if node.dir === "col"}
-  <div class="flex min-h-0 w-full flex-col" style:outline={markerOutline(node.marker)}>
+  <div class="flex min-h-0 w-full flex-col" style={markerStyle(node.marker)}>
     {#each node.children as child, i (i)}
       {@const key = keyOf(i, child.node.id)}
       <div
@@ -51,11 +72,19 @@
     {/each}
   </div>
 {:else}
-  <div class="flex min-h-0 h-full flex-row" style:outline={markerOutline(node.marker)}>
+  <div class="flex min-h-0 h-full flex-row" style={markerStyle(node.marker)}>
     {#each node.children as child, i (i)}
       {@const key = keyOf(i, child.node.id)}
       {#if child.collapsible}
         {@const open = isOpen(i, child.node.id)}
+        <!--
+          A diagnostic marker on a collapsible child is drawn on the SCROLL
+          CONTAINER below — the sidebar's fixed, opaquely-painted viewport —
+          NOT on the child's own root element: that root renders INSIDE the
+          scroll container, so an outline there would wrap the full
+          scrollable content and scroll along with it instead of framing the
+          visible panel.
+        -->
         <!-- svelte-ignore a11y_no_static_element_interactions -->
         <aside
           class="flex shrink-0 flex-col overflow-x-hidden transition-[width] duration-300 ease-out max-lg:absolute max-lg:inset-y-0 max-lg:right-0 max-lg:z-30 lg:relative"
@@ -65,7 +94,8 @@
         >
           <div
             class="flex h-full flex-col gap-2 overflow-y-auto bg-base-100 pt-3 pr-3 pb-3 transition-transform duration-300 ease-out"
-            style="width: {child.basis ?? 320}px; transform: translateX({open ? '0' : '100%'})"
+            style={scrollViewportStyle(child.basis, open, child.node.marker)}
+            data-testid="sidebar-scroll-viewport"
           >
             <TileNode node={child.node} path={[...path, i]} {ports} {collapse} {onEdit} />
           </div>