feat(panels): open documents with top-level layers collapsed

A fully expanded tree stops answering the one question the LayerPanel
exists for. A six-slide deck opens as ~90 rows, so the boards themselves
scroll off the panel and the user cannot see what the document contains
without scrolling past every leaf of slide one.

Collapse only top-level containers on load: leaves have nothing to hide
and would render a dead disclosure arrow. `collapsed_layers` is view-only
state — not serialized, not in the undo snapshot, no history push — so
this changes what the panel shows on open and nothing about the document.

Claude-Session: https://claude.ai/code/session_01FqKQqNj8exYwopGDpYUU7x

crates/op-host-services/src/doc_io/load.rs

@@ -130,9 +130,35 @@ fn finish_loaded_state(mut state: EditorState, meta: Option<EditorMeta>) -> Edit
     } else {
         state.ui.active_page_index = first_page_with_content(&state);
     }
+    collapse_top_level_layers(&mut state);
     state
 }
 
+/// Start every top-level frame collapsed in the LayerPanel.
+///
+/// A fully expanded tree is unreadable the moment a document has depth: a
+/// six-slide deck opens as ~90 rows and the boards themselves scroll off the
+/// panel, so the one thing the list is for — seeing what the document
+/// contains — is the first thing lost. Collapsed roots show the six slides
+/// and let the user open the one they want.
+///
+/// `collapsed_layers` is view-only state: not serialized, not in the undo
+/// snapshot, and toggling it pushes no history. So this changes what the
+/// panel shows on open and nothing about the document.
+fn collapse_top_level_layers(state: &mut EditorState) {
+    let ids: Vec<op_editor_core::NodeId> = state
+        .active_children()
+        .iter()
+        .filter(|node| {
+            // Only containers collapse; a leaf has nothing to hide and would
+            // just render a dead disclosure arrow.
+            op_editor_core::PenNodeExt::children(*node).is_some_and(|kids| !kids.is_empty())
+        })
+        .map(|node| op_editor_core::NodeId::new(op_editor_core::PenNodeExt::id_str(node)))
+        .collect();
+    state.editor_ui.collapsed_layers.extend(ids);
+}
+
 fn first_page_with_content(state: &EditorState) -> usize {
     let Some(pages) = state.doc.pages.as_ref() else {
         return 0;

crates/op-host-services/src/doc_io/load_report_tests.rs

@@ -105,3 +105,39 @@ fn reports_only_adopted_sidecar_and_reliable_editor_meta_inference() {
     let _ = std::fs::remove_file(inferred);
     let _ = std::fs::remove_file(ordinary);
 }
+
+/// Top-level frames open collapsed in the LayerPanel.
+///
+/// A six-slide deck expands to ~90 rows, pushing the boards themselves off
+/// the panel — the list stops answering "what is in this document", which is
+/// the only question it exists for.
+#[test]
+fn loading_collapses_top_level_frames_but_not_leaves() {
+    let source = serde_json::json!({
+        "version": "1.0",
+        "children": [
+            {
+                "type": "frame", "id": "board-1", "name": "01", "width": 1920, "height": 1080,
+                "children": [{"type": "text", "id": "t1", "content": "hi"}]
+            },
+            {
+                "type": "frame", "id": "board-2", "name": "02", "width": 1920, "height": 1080,
+                "children": [{"type": "text", "id": "t2", "content": "hi"}]
+            },
+            // A childless top-level node: collapsing it would only render a
+            // disclosure arrow that does nothing.
+            {"type": "rectangle", "id": "loose", "name": "Loose", "width": 10, "height": 10}
+        ]
+    })
+    .to_string();
+
+    let state =
+        super::load_editor_state_from_source(&source, op_editor_core::Locale::EnUs).expect("loads");
+    let collapsed = &state.editor_ui.collapsed_layers;
+    assert!(collapsed.contains(&op_editor_core::NodeId::new("board-1")));
+    assert!(collapsed.contains(&op_editor_core::NodeId::new("board-2")));
+    assert!(
+        !collapsed.contains(&op_editor_core::NodeId::new("loose")),
+        "a leaf has nothing to collapse"
+    );
+}