Copy and paste items between strategies on the same map

lib/collab/page_made_here.dart

@@ -23,7 +23,7 @@ List<StrategyOp> pageCopyContentOps({
   final newLineupId = _sharedCopyIds();
   return [
     for (final element in elements)
-      _copiedElement(pageId, element, _copyId(element.publicId)),
+      _copiedElement(pageId, element, newStorableCopyId(element.publicId)),
     for (final lineup in lineups)
       _copiedLineupGroup(pageId, lineup, newLineupId),
   ];
@@ -67,21 +67,11 @@ RemotePageSnapshot pageMadeHere({
   );
 }
 
-/// The longest root a copy id keeps, URL-encoded, as the server's
-/// (convex/lib/pageCopyId.ts): a longer one would make the op's storage
-/// key too long to keep, so that copy gets a plain id.
-const _maxKeptRootLength = 80;
-
-String _copyId(String id) =>
-    Uri.encodeComponent(pageCopyRoot(id)).length > _maxKeptRootLength
-        ? const Uuid().v4()
-        : newPageCopyId(id);
-
 /// One copy id per id, shared by every lineup group copied: a spot or
 /// lineup named twice is the same copy both times.
 String Function(String id) _sharedCopyIds() {
   final ids = <String, String>{};
-  return (id) => ids[id] ??= _copyId(id);
+  return (id) => ids[id] ??= newStorableCopyId(id);
 }
 
 ElementAddOp _copiedElement(String pageId, DrawnRow element, String id) {

lib/const/line_provider.dart

@@ -377,9 +377,12 @@ class LineUpGraph {
   /// The lineups [linkIds] and the spots they aim at, ready to be put on
   /// another page, each under a new id that carries its original's (see
   /// page_copy_id.dart). Spots they share stay shared in the copy.
-  LineUpGraph copyOfLinks(Set<String> linkIds) {
+  LineUpGraph copyOfLinks(
+    Set<String> linkIds, {
+    String Function(String id) newId = newPageCopyId,
+  }) {
     final newIds = <String, String>{};
-    String renamed(String id) => newIds[id] ??= newPageCopyId(id);
+    String renamed(String id) => newIds[id] ??= newId(id);
     final part = linksWithSpots(linkIds);
     return LineUpGraph(
       origins: [
@@ -1398,6 +1401,20 @@ class LineUpProvider extends Notifier<LineUpState> {
     }
   }
 
+  /// Adds [copies] (a paste: lineups whose ids are all new) as one undoable
+  /// addition.
+  void addCopies(LineUpGraph copies) {
+    if (!ref.canEditPages || copies.links.isEmpty) return;
+    _apply(from: LineUpGraph.empty, to: copies);
+    _record(
+      LineUpGraphAction(
+        type: ActionType.addition,
+        id: copies.links.first.id,
+        after: copies.deepCopy(),
+      ),
+    );
+  }
+
   // --- Deletions -----------------------------------------------------------
 
   void deleteLink(String linkId) {

lib/const/page_copy_id.dart

@@ -34,3 +34,15 @@ String pageCopyRoot(String id) {
 /// A fresh id for a copy of [id] on another page.
 String newPageCopyId(String id) =>
     '${pageCopyRoot(id)}$_copyMark${const Uuid().v4()}';
+
+/// The longest root a copy id keeps, URL-encoded, as the server's
+/// (convex/lib/pageCopyId.ts): a longer one would make the op's storage
+/// key too long to keep, so that copy gets a plain id.
+const _maxKeptRootLength = 80;
+
+/// [newPageCopyId], or a plain id when [id]'s root is too long for the copy
+/// to be stored to send (only an item imported with an unusually long id).
+String newStorableCopyId(String id) =>
+    Uri.encodeComponent(pageCopyRoot(id)).length > _maxKeptRootLength
+        ? const Uuid().v4()
+        : newPageCopyId(id);