Custom View: native sticky header, one-line Excel-style header; fixes

- Custom View header now uses the browser's sticky positioning like the
  other tables, so it no longer lags or bounces while scrolling. Because a
  sideways-scrolling box prevents sticky headers, the table is clipped and
  shifted sideways by a scrollbar pinned to the bottom of the window
  (shown only when the columns are wider than the page; trackpad,
  shift+wheel and touch swipes drive it too)
- Custom View header on one line like the other tables: drag grip, name,
  the combined sort/filter dropdown and copy icon (remove on hover)
- Workbench stack text boxes size to their contents on load and when the
  page is shown, not only while typing
- Demo sandbox waits for the worker's first sweep after a container
  restart instead of treating every device as unknown

web/demo/demo.js

@@ -555,6 +555,12 @@
             const tracked = await visitorTracked();
             const applied = s.appliedTracked || tracked;
             const snap = await serverSnapshot();
+            if (!snap.rows.length) {           // container just started: nothing to sweep against yet
+                w([['[Waiting for the first worker sweep to finish...]', 'DarkGray']]);
+                Sim.again = kind;
+                await sleep(5000);
+                return;
+            }
             const before = new Set(Object.keys(parseRoster(applied).tagMap));
             const { rows } = buildRows(tracked, snap.rows, snap.adv);
             const { tagMap } = parseRoster(tracked);
@@ -700,14 +706,16 @@
         'GET Data.json': async () => {
             const s = Store.state;
             const snap = await serverSnapshot();
-            if (!s.appliedTracked) return jsonResponse(snap.data);
+            // Until the worker's first sweep there are no rows to merge with: pass through so
+            // the dashboard shows its "Waiting for Worker..." placeholders
+            if (!s.appliedTracked || !snap.rows.length) return jsonResponse(snap.data);
             const { rows } = buildRows(s.appliedTracked, snap.rows, snap.adv);
             return jsonResponse({ LastUpdated: snap.data.LastUpdated || psStamp(), Devices: rows });
         },
         'GET Advanced-Cache.json': async () => {
             const s = Store.state;
             const snap = await serverSnapshot();
-            if (!s.appliedTracked) return jsonResponse(snap.adv);
+            if (!s.appliedTracked || !snap.rows.length) return jsonResponse(snap.adv);
             const { adv } = buildRows(s.appliedTracked, snap.rows, snap.adv);
             for (const [k, val] of Object.entries(adv)) if (val) snap.adv[k] = val;
             return jsonResponse(snap.adv);