diff --git a/CLAUDE.md b/CLAUDE.md
index 4761478..535f36c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -272,6 +272,21 @@ gone:
 
 `split_full` is still *read* on load, for layouts saved under the old scheme.
 
+### Auto-pause is off
+
+`<perspective-viewer>` auto-pauses by default: an `IntersectionObserver` on
+itself plus the document's `visibilitychange` drive `AutoPauseState::apply()`,
+and pausing **deletes the view** (`session.set_pause(true)` →
+`view_sub.take().delete()`). Returning to the tab is therefore not a redraw but
+`restore_and_render()` — a fresh view and a fresh traversal of the whole grain,
+which on a large one is a multi-second chug on every tab switch, and takes any
+per-node expansion with it.
+
+`initViewer()` calls `viewer.setAutoPause(false)` right after `viewer.load()`.
+Nothing updates the table while the tab is hidden — every operation is driven
+from this page — so the pause bought nothing and cost a rebuild. The view is now
+held while the tab is backgrounded.
+
 ### Still not solved: per-node expansion
 
 Expanding one specific branch is view state with no config representation, and
diff --git a/ui/src/views/Forecast.jsx b/ui/src/views/Forecast.jsx
index 24847a7..c7434bb 100644
--- a/ui/src/views/Forecast.jsx
+++ b/ui/src/views/Forecast.jsx
@@ -259,12 +259,6 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
   // the current selection, readable from event handlers that were registered once
   const slicesRef      = useRef([])
 
-  // Perspective's ROLLUP view contains every level of the hierarchy; view.set_depth()
-  // is the only thing hiding the deeper ones, and it lives on the view rather than in
-  // the saved config. The viewer rebuilds its view when it redraws — which its
-  // Intersection/ResizeObserver triggers when you switch back to the tab — and the
-  // fresh view has no depth set, so the whole tree appears expanded. Re-apply the
-  // depth we last set once the redraw has settled.
   // perspective-click fires as a CustomEvent with no modifier state of its own,
   // and carries no cell coordinates either, so record both from the mousedown that
   // precedes it. The grid lives in a shadow root, so the real target is the head of
@@ -822,6 +816,22 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
       // that occurs when viewer.load(worker) + restore({ table: name }) can't resolve
       // the named table in time.
       await viewer.load(tableRef.current)
+
+      // Auto-pause is on by default, and pausing *deletes the view*:
+      // AutoPauseState::apply() fires on visibilitychange and on the viewer's own
+      // IntersectionObserver, and set_pause(true) does
+      // `view_sub.take().delete()`. Coming back is therefore not a redraw but
+      // `restore_and_render()` -- a fresh view, a fresh traversal of the whole
+      // grain -- which is the multi-second chug on every tab switch, and why any
+      // per-node expansion is gone when you return.
+      //
+      // Nothing updates the table while the tab is hidden (operations are driven
+      // from this page), so the pause buys nothing here and costs a rebuild.
+      // Guarded: it is a method on the custom element, absent on older builds.
+      try { if (viewer.setAutoPause) await viewer.setAutoPause(false) } catch (err) {
+        console.error('[setAutoPause]', err)
+      }
+
       viewer.setAttribute('theme', dark ? 'Pro Dark' : 'Pro Light')
       if (!hideSplitTotal()) setTimeout(hideSplitTotal, 400)