swarm-ui: the job graph shows placeholder rows in the shape of a job tree while it loads

JobqGraph takes an optional loading element, so the dashboard keeps its text
and the swarm UI's jobs tab passes a placeholder built from the graph's own
node and row classes: a root with two children and a second root, with the
real indent and rail.

docs/2-handbook/swarm/ui.md

@@ -583,7 +583,8 @@ previous repo's issues under the error.
 
 ### Builds
 
-The builds sit in a card under the job graph on the **jobs** tab. Both cards start
+The builds sit in a card under the job graph on the **jobs** tab. While the graph
+loads for the first time, it shows grey placeholder rows in the shape of a job tree. Both cards start
 shut, and each reads nothing until you open it; a link to
 a build opens the builds card. The old `/builds` address redirects there and
 keeps its query.

frontend/packages/shared/src/jobq-graph/JobqGraph.tsx

@@ -28,6 +28,7 @@
 // loader is `text` (for unrelated shadow-DOM components' CSS-as-string
 // needs), and that loader is global per call, not per-module.
 
+import type { ComponentChildren } from "preact";
 import { useState, useEffect, useCallback, useRef } from "preact/hooks";
 
 // Mirrors `hive_jobq_wire::StateSchema` verbatim (variant names, no
@@ -283,6 +284,9 @@ export interface JobqGraphProps {
   onUpdate?: (nodes: GraphNode[]) => void;
   onCancel?: (id: number) => void;
   refreshToken?: number;
+  /** What shows while the first answer loads. A host that has a placeholder in
+   * the shape of its graph passes it; the default is a line of text. */
+  loading?: ComponentChildren;
 }
 
 // `refreshToken` is not read anywhere in the body — its only job is to
@@ -295,6 +299,7 @@ export function JobqGraph({
   onUpdate,
   onCancel,
   refreshToken = 0,
+  loading,
 }: JobqGraphProps) {
   const [selectedStates, setSelectedStates] = useState<Set<NodeState>>(
     () => new Set(ALL_STATES.filter((s) => !DEFAULT_HIDDEN_STATES.has(s))),
@@ -344,7 +349,7 @@ export function JobqGraph({
         {error ? (
           <p class="jg-error-msg">fetch failed: {error}</p>
         ) : nodes === null ? (
-          <p class="jg-empty">loading…</p>
+          (loading ?? <p class="jg-empty">loading…</p>)
         ) : !nodes.length ? (
           <p class="jg-empty">empty</p>
         ) : (

frontend/packages/swarm-ui/src/pages/JobsPage.tsx

@@ -18,10 +18,12 @@
 // fetch — both `JobqGraph`/`JobqRollup` already accept that prop
 // (bumping it is their documented "refetch now" lever) since the
 // dashboard's own rebuild-queue view needed the same thing first.
+import type { ComponentChildren } from "preact";
 import { useState } from "preact/hooks";
 import { JobqGraph } from "@hive/shared/jobq-graph.js";
 import { JobqRollup } from "@hive/shared/jobq-rollup.js";
 import { LazyDisclosure } from "../ui/disclosure/LazyDisclosure.js";
+import { Skeleton, SkeletonRegion } from "../ui/skeleton/Skeleton.js";
 import { Toolbar } from "../ui/toolbar/Toolbar.js";
 import { BuildsCard } from "./builds/BuildsPage.js";
 import { ReviewPage } from "./review/ReviewPage.js";
@@ -37,6 +39,39 @@ import "./JobsPage.css";
 // just solve staleness rather than require an opt-in every visit.
 const DEFAULT_INTERVAL_MS: RefreshIntervalMs = 30_000;
 
+// The graph's twin while it loads: the graph's own node and row classes, so the
+// rows and the indent of the children are the real ones, with a state glyph and a
+// label as placeholders. Two roots, the first with two children.
+function SkeletonNode({
+  label,
+  children,
+}: {
+  label: string;
+  children?: ComponentChildren;
+}) {
+  return (
+    <div class="jg-node">
+      <div class="jg-row">
+        <Skeleton shape="circle" width="1em" height="1em" />
+        <Skeleton width={label} />
+      </div>
+      {children}
+    </div>
+  );
+}
+
+function JobsGraphSkeleton() {
+  return (
+    <SkeletonRegion label="the jobs">
+      <SkeletonNode label="14em">
+        <SkeletonNode label="10em" />
+        <SkeletonNode label="12em" />
+      </SkeletonNode>
+      <SkeletonNode label="9em" />
+    </SkeletonRegion>
+  );
+}
+
 function JobsBody() {
   const [intervalMs, setIntervalMs] =
     useState<RefreshIntervalMs>(DEFAULT_INTERVAL_MS);
@@ -62,7 +97,11 @@ function JobsBody() {
         />
       </Toolbar>
       <JobqRollup endpoint="/api/jobq/rollup" refreshToken={refreshToken} />
-      <JobqGraph endpoint="/api/jobq/graph" refreshToken={refreshToken} />
+      <JobqGraph
+        endpoint="/api/jobq/graph"
+        refreshToken={refreshToken}
+        loading={<JobsGraphSkeleton />}
+      />
     </>
   );
 }