diff --git a/ui/src/views/Forecast.jsx b/ui/src/views/Forecast.jsx
index 2a7d9f9..384f784 100644
--- a/ui/src/views/Forecast.jsx
+++ b/ui/src/views/Forecast.jsx
@@ -346,14 +346,14 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
     let settle = null
     const onObserved = (ev) => {
       const d = ev.detail || {}
-      // Losing intersection, or collapsing to zero size, is the element going
-      // away: record it, because that is when the view gets discarded.
-      const leaving = d.entries?.some(e =>
-        e.isIntersecting === false || (e.width === 0 && e.height === 0))
+      // Only intersection counts as going away. A zero size is a transient of
+      // layout -- opening Perspective's own settings sidebar collapses the
+      // datagrid for a frame -- and treating it as a departure meant the return
+      // re-applied depth, so adjusting the layout snapped the tree back.
+      const leaving = d.entries?.some(e => e.isIntersecting === false)
       if (leaving) { wentAwayRef.current = true; return dbg(`observer ${d.kind} -> viewer went away`) }
 
-      // A resize with a non-zero size is an ordinary reflow. It is not a
-      // restoration, so it must not re-apply anything.
+      // Everything else is a reflow. Not a restoration, so it restores nothing.
       if (!wentAwayRef.current) return
 
       clearTimeout(settle)
@@ -875,7 +875,17 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
           // it redefines the hierarchy. Ours is a collapse, and must not overwrite it.
           if (!collapsingRef.current) {
             const live = await viewer.save()
-            adoptSplit(live.split_by || [], (live.split_by || []).length)
+            const next = live.split_by || []
+            const full = splitFullRef.current || []
+            // This event fires for anything in the config -- opening the settings
+            // sidebar included -- not only for split_by. While collapsed, the live
+            // split_by is a prefix of the full hierarchy, and adopting it would
+            // record the truncation as the hierarchy and make the deeper levels
+            // unreachable. A genuine rearrangement is not a prefix of what we hold.
+            const isCollapsedPrefix =
+              next.length < full.length && next.every((c, i) => c === full[i])
+            if (!isCollapsedPrefix) adoptSplit(next, next.length)
+            else dbg(`config-update: split_by is our collapse (${next.length}/${full.length}), keeping the full hierarchy`)
           }
           const cfg = await captureConfig()
           if (cfg) await persistLayout(vid, cfg)