fix(paper): reset a reused viewport GPU slot so the survivor keeps its content

Pipeline slots are addressed by list index, but the renderer drops
off-canvas viewports from the list, so a slot can be reused by a
different viewport across frames — e.g. when the first paper viewport
scrolls off the canvas the second slides into its slot and inherits its
stale, differently frustum-culled GPU buffers. The text upload is gated
on a geometry-epoch change (unaffected by the index shift), so the
survivor rendered the gone viewport's text and its own vanished.

Give each viewport a stable identity, remember which one last used a
slot, and reset all cache keys when the occupant changes so wires, text,
hatches, meshes and Face3D re-upload for the new viewport. No cost in the
steady state — the reset fires only when a viewport enters/leaves view.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pipeline/mod.rs

@@ -228,6 +228,12 @@ pub struct Pipeline {
     /// `render` skips the (per-pixel, GPU-dominating) hatch pass this frame. The
     /// scene-render cache holds the full-quality frame once the view settles.
     pub skip_hatch_frame: bool,
+    /// Stable identity of the viewport that last used this (index-addressed)
+    /// pipeline slot. The renderer's viewport list drops off-canvas viewports,
+    /// so a slot can be reused by a *different* viewport across frames; when the
+    /// occupant changes, all cache keys above belong to the previous one and are
+    /// reset so every buffer re-uploads. `u64::MAX` = never used.
+    pub slot_id: u64,
 }
 
 impl Pipeline {
@@ -1226,6 +1232,7 @@ impl Pipeline {
             render_sig: u64::MAX,
             skip_geometry: false,
             skip_hatch_frame: false,
+            slot_id: u64::MAX,
         }
     }
 

src/scene/view/render.rs

@@ -31,6 +31,11 @@ pub struct CameraState {
 /// own inner `Pipeline` instance drawn into its own rectangle.
 #[derive(Debug)]
 pub struct ViewportData {
+    /// Stable identity of the source viewport (its entity handle / tile index /
+    /// sheet role). The renderer addresses pipeline slots by list index but
+    /// drops off-canvas viewports, so this lets the slot detect when it has been
+    /// reused by a different viewport and reset its (index-addressed) caches.
+    pub(in crate::scene) instance_id: u64,
     pub(in crate::scene) wires: Arc<Vec<WireModel>>,
     /// Live command-preview / interim / grip-drag overlay wires. Kept out of
     /// the main `wires` buffer so a drag re-uploads only this small set each
@@ -224,6 +229,24 @@ impl shader::Primitive for Primitive {
 
         for (i, vp) in self.viewports.iter().enumerate() {
             let inner = &mut pipeline.inners[self.base_slot + i];
+            // Pipeline slots are addressed by list index, but off-canvas
+            // viewports are dropped from the list — so a slot can be reused by a
+            // DIFFERENT viewport across frames (e.g. the first viewport scrolls
+            // off the canvas and the second slides into its slot). When that
+            // happens every cache key below belongs to the previous occupant;
+            // reset them so wires, text, hatches, meshes and Face3D all
+            // re-upload for the new viewport instead of showing the previous
+            // one's (differently frustum-culled) content — which otherwise makes
+            // the surviving viewport's text/geometry vanish.
+            if inner.slot_id != vp.instance_id {
+                inner.slot_id = vp.instance_id;
+                inner.cached_epoch = (u64::MAX, u64::MAX, u64::MAX);
+                inner.cached_wire_id = u64::MAX;
+                inner.cached_selection = (u64::MAX, u64::MAX);
+                inner.cached_mesh_key = (u64::MAX, u64::MAX);
+                inner.cached_face3d_key = (u64::MAX, false);
+                inner.render_sig = u64::MAX;
+            }
             // The MSAA / depth / resolve textures are always sized to the
             // FULL viewport rectangle (not the on-canvas-visible portion)
             // so the camera matrices render at consistent aspect / scale.
@@ -1151,7 +1174,18 @@ impl Scene {
             pv.extend_from_slice(&self.preview_text);
             Arc::new(pv)
         };
+        // Stable per-viewport identity (tagged so tile / sheet / content /
+        // implicit-model instances never collide), so a reused pipeline slot
+        // can tell it changed occupant and reset its caches.
+        let instance_id: u64 = if let Some(t) = inst.tile_idx {
+            0x1000_0000_0000_0000 | (t as u64)
+        } else if inst.paper_sheet {
+            0x2000_0000_0000_0000
+        } else {
+            0x3000_0000_0000_0000 | inst.handle.value()
+        };
         Some(ViewportData {
+            instance_id,
             wires: all_wires,
             preview_wires,
             face3d_wires,