Live collaboration with Yjs over ActionCable

All editors now work on a server document at the same time instead of
taking turns behind an edit lock.

- The document's content is a sequence of Yjs updates (document_updates);
  DocumentChannel stores and relays them immediately, checks access on
  subscribe and again whenever links or memberships change.
- app/frontend/collab/: the store stays position-based; a binding maps it
  to a Yjs form with stable row/column ids, so a row inserted by one
  person doesn't misplace what another is typing. Remote changes are
  applied only after pending local ones went into Yjs.
- Per-person undo (Y.UndoManager), presence via awareness: the others'
  selections as coloured boxes with names, avatars in the toolbar.
- Existing JSON documents convert on first open, deterministically, so
  two people opening one at once end up with the same Yjs state.
- Editors compact the stored updates after 200 and keep a JSON snapshot
  current for export and duplicating. Edit lock and polling are gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/channels/document_channel_test.rb

@@ -0,0 +1,94 @@
+require "test_helper"
+
+class DocumentChannelTest < ActionCable::Channel::TestCase
+  setup do
+    @anna = create_user("anna@example.com")
+    @bob = create_user("bob@example.com")
+    @doc = create_document(@anna)
+  end
+
+  def b64(s) = Base64.strict_encode64(s)
+  def last = transmissions.last
+
+  test "rejects without access" do
+    stub_connection current_user: @bob
+    subscribe id: @doc.id
+    assert subscription.rejected?
+  end
+
+  test "init sends the JSON while there are no updates yet, then the updates" do
+    stub_connection current_user: @anna
+    subscribe id: @doc.id, key: "a"
+    assert subscription.confirmed?
+    assert_equal "owner", last["role"].to_s
+    assert_equal DOC_JSON, last["content"]
+    assert_empty last["updates"]
+
+    @doc.updates.create!(data: "\x01\x02")
+    subscribe id: @doc.id, key: "a"
+    assert_nil last["content"]
+    assert_equal [ b64("\x01\x02") ], last["updates"].map { _1["data"] }
+  end
+
+  test "an editor's update is stored and passed on to everyone" do
+    stub_connection current_user: @anna
+    subscribe id: @doc.id, key: "a"
+    assert_broadcasts(DocumentChannel.broadcasting_for(@doc), 1) do
+      perform :update, data: b64("abc")
+    end
+    assert_equal [ "abc" ], @doc.updates.pluck(:data)
+  end
+
+  test "a viewer (link) can't change anything but may send awareness" do
+    link = @doc.share_links.create!(role: :viewer, created_by: @anna)
+    stub_connection current_user: nil
+    subscribe id: @doc.id, token: link.token, key: "v"
+    assert subscription.confirmed?
+    perform :update, data: b64("x")
+    perform :compact, state: b64("x"), ids: [], snapshot: DOC_JSON
+    assert_equal 0, @doc.updates.count
+    assert_broadcasts(DocumentChannel.broadcasting_for(@doc), 1) do
+      perform :awareness, data: b64("cursor")
+    end
+  end
+
+  test "compact replaces the listed updates by one state and stores the snapshot" do
+    a = @doc.updates.create!(data: "1")
+    b = @doc.updates.create!(data: "2")
+    keep = @doc.updates.create!(data: "3")
+    stub_connection current_user: @anna
+    subscribe id: @doc.id, key: "a"
+    snapshot = { version: 2, sheets: [ { id: "s9", name: "Neu", objects: [] } ] }.to_json
+    perform :compact, state: b64("12"), ids: [ a.id, b.id ], snapshot: snapshot
+    assert_equal %w[3 12], @doc.updates.order(:id).pluck(:data)
+    assert keep.reload
+    assert_equal snapshot, @doc.reload.content
+  end
+
+  test "invalid snapshots are ignored" do
+    stub_connection current_user: @anna
+    subscribe id: @doc.id, key: "a"
+    perform :snapshot, content: "[1]"
+    assert_equal DOC_JSON, @doc.reload.content
+  end
+
+  test "losing access mid-session" do
+    DocumentMembership.grant!(document: @doc, user: @bob, role: :editor, source: :invite)
+    stub_connection current_user: @bob
+    subscribe id: @doc.id, key: "b"
+    @doc.document_memberships.delete_all
+    subscription.send(:revalidate)
+    assert_equal "revoked", last["type"]
+    perform :update, data: b64("late")
+    assert_equal 0, @doc.updates.count
+  end
+
+  test "role change is announced" do
+    DocumentMembership.grant!(document: @doc, user: @bob, role: :editor, source: :invite)
+    stub_connection current_user: @bob
+    subscribe id: @doc.id, key: "b"
+    @doc.document_memberships.update_all(role: DocumentMembership.roles[:viewer])
+    subscription.send(:revalidate)
+    assert_equal({ "type" => "role", "role" => "viewer" }, last.slice("type", "role").transform_values(&:to_s))
+  end
+end