Write down what auto-pause actually does

It reads like a paint optimisation and is not: pausing deletes the view, so
becoming visible again is a full rebuild. dataflow embeds the same viewer and
would hit the same stall, so it belongs in the shared reference next to the
other things that cost us a day to find.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

PERSPECTIVE.md

@@ -268,6 +268,32 @@ it would retire the `if(...)`-expression workaround. It costs the d3fc charts, t
   column; both need validating, and the entry should be dropped rather than the layout.
   Add this guard as part of adopting §3a, not after.
 
+### Auto-pause deletes the view — turn it off for a static pivot
+
+`<perspective-viewer>` auto-pauses by default: an `IntersectionObserver` on itself
+(scrolled out of the viewport, `display: none`) combined with the document's
+`visibilitychange` (backgrounded tab, minimized window). "Pause" is not a paint
+optimisation — `session.set_pause(true)` runs `view_sub.take().delete()`, so the
+**view object is destroyed**. Becoming visible again calls
+`restore_and_render(…, ViewerConfigUpdate::default())`: a new view and a full
+traversal, every time.
+
+For a viewer streaming live updates nobody is watching, that is the right trade.
+For a pivot over a large static table it is the wrong one — on pf_app's
+`fc_osm_skinny_29` grain it is a multi-second stall on every tab switch, and it
+silently discards per-node expand/collapse (§"Not fixed by any of this"), which
+has no config representation and so cannot be restored.
+
+```js
+await viewer.load(table)
+try { if (viewer.setAutoPause) await viewer.setAutoPause(false) } catch {}
+```
+
+Guard the call: it is a method on the custom element and absent on older builds.
+Leave auto-pause **on** where the table is fed by a live stream the user does not
+need to have kept up with while away. This is long-standing viewer behaviour, not
+a 5.x regression — worth knowing before blaming a rebuild on your own code.
+
 ---
 
 ## 6. Build & deploy (target)