Decocms Home Widgets

deco CMS Home Widgets

MCP tools can render a small React app ("MCP-App") as a widget/tile on the deco home ("What's on your mind" screen). This skill explains how the home sizes those tiles and how to make a widget that looks right at every size and state.

When to use this skill

• Building or fixing a home widget for an MCP (the small resizable cards on the home).
• A widget's content is being clipped, has no scroll, wastes space, or doesn't adapt when resized.
• You need to register a new widget resource.

Reference implementation: tanstack-migrator/web/tools/widget-queue/index.tsx (responsive 2-column queue + suggestions with internal scroll) and tanstack-migrator/web/tools/widget-active/index.tsx (compact single card).

How the tile works (READ THIS FIRST)

The home controls the tile size, not the MCP. The user resizes each tile with a grid picker ("Size", columns × rows, up to 4×5). There is no API to declare a default size from the MCP — assume the widget can be handed any width × height.

The home wraps your widget in a tile that is roughly:

div.relative.flex.h-full.w-full.flex-col.p-3        (larger tiles)
div.flex.min-h-0.w-full.flex-1.flex-col.overflow-hidden   (smaller tiles)

Two consequences that drive every rule below:

1. The tile is h-full w-full — your widget should fill it, not hug content.
2. The tile is overflow-hidden — anything taller than the tile is clipped (no scrollbar) unless you add an internal scroll region.

Observed tile sizes (px)

≈ 316px per column, ≈ 100px per row. Widths and heights combine freely (e.g. 1×2, 2×2, 3×3, 4×5), so design for a range, not fixed dimensions.

Golden rules

1. Root fills the tile: className="flex h-full min-h-0 flex-col". Never rely on natural content height — tall content gets clipped by the tile's overflow-hidden.

2. Scroll inside: put lists in a region with flex-1 min-h-0 overflow-y-auto. min-h-0 is required or the flex child refuses to shrink and the scroll never engages.

3. Breakpoints via ResizeObserver, NOT media queries. The tile is not the viewport — @media/sm: react to the window, not the tile. Measure the tile:

   function useWide(ref: React.RefObject<HTMLElement | null>, px: number) {
     const [wide, setWide] = useState(false);
     useEffect(() => {
       const el = ref.current; if (!el) return;
       const ro = new ResizeObserver((es) => { for (const e of es) setWide(e.contentRect.width >= px); });
       ro.observe(el); return () => ro.disconnect();
     }, [ref, px]);
     return wide;
   }
   

   (Tailwind v4 @container queries also work if you mark the root @container.)

4. Adapt columns to width. e.g. ≥520px → two columns side by side; below → stack them in one scroll column. Give empty/low-value sections less room (grid-cols-[0.8fr_1.2fr]) so the useful content isn't cramped.

5. Every state fills too: loading / error / empty use flex flex-1 items-center justify-center so they center in the tile at any size.

6. Avoid double padding: the tile already applies p-3 at larger sizes. Keep a single modest padding on your root; don't nest another p-3 inside.

7. Icons on a dark tile: transparent/dark logos vanish on the dark background. Put remote favicons on a fixed light background (bg-white object-contain p-0.5) and fall back through logo → screenshot → colored letter-avatar (never an empty gray box — some icon endpoints return a blank 200 that won't fire onError).

Registering a widget (decocms/mcps pattern)

Five wiring points (see tanstack-migrator/ for a live example):

1. Constants — a resource URI in server/constants.ts: export const WIDGET_QUEUE_RESOURCE_URI = "ui://<mcp>/widget-queue";
2. Tool — in server/tools/widgets.ts, a createTool that returns the widget's data and links the UI via _meta: { ui: { resourceUri: WIDGET_QUEUE_RESOURCE_URI } } and annotations: { readOnlyHint: true }. Register it in server/tools/index.ts.
3. Resource — in server/resources/index.ts, createMcpAppResource({ uri, name, description, htmlFile: "widget-queue.html" }); add it to the resources: [...] array in server/main.ts.
4. Vite entry — add the HTML to MCP_APP_ENTRIES in vite.config.ts and to the build:web script in package.json (MCP_APP_ENTRY=widget-queue … vite build).
5. HTML + component — widget-queue.html (root div + <script src=./web/tools/widget-queue/main.tsx>) and the React component in web/tools/widget-queue/index.tsx, fetching data with usePollingTool<T>(TOOL_ID, {}, intervalMs) from web/hooks/use-tool.ts.

Checklist before shipping a widget

• Root is flex h-full min-h-0 flex-col (fills the tile).
• Long lists live in flex-1 min-h-0 overflow-y-auto (scroll, not clip).
• Layout switches on tile width via ResizeObserver/@container, not media queries.
• loading / error / empty states center with flex-1.
• Looks right at 1×2, 2×2, 3×3, 4×5 and the minimum tile.
• No double p-3; icons readable on the dark background with a real fallback.
• Widget wired: constants → tool _meta.ui → resource → main.ts → vite entry → html.