feat: canvas harness

README.md

@@ -66,6 +66,10 @@ Everything on the board is a node:
 
 https://github.com/user-attachments/assets/ad5de9f4-6f44-43a2-b59a-5279232d7f60
 
+## Canvas engine
+
+The board is built on [canvas-harness](https://github.com/winlp4ever/canvas-harness), a canvas-rendered node-graph library we maintain separately. Boards can hold thousands of nodes and still pan, zoom, and edit smoothly — comparable to tldraw and Excalidraw, and on par with hosted tools like Miro or FigJam.
+
 ## Agent layer
 
 Built on the OpenAI Agents SDK, with board-aware tools wired in:

backend/topix/datatypes/property.py

@@ -174,6 +174,15 @@ class Position(BaseModel):
 
     type: Literal[PropertyType.POSITION] = PropertyType.POSITION
     position: Position | None = None
+    # When the position belongs to an edge endpoint AND the link's
+    # source/target resolves to an attached node, callers may interpret
+    # `position` as a node-local offset (relative to the node's
+    # top-left, pre-rotation) instead of an absolute world coordinate.
+    # Default `False` keeps the legacy world-coord interpretation for
+    # existing rows so no data migration is needed. Newer clients set
+    # this to True when saving attached endpoints so edges that move
+    # with their node don't require cascading updates.
+    is_local_offset: bool = False
 
 
 class SizeProperty(Property):

backend/topix/store/graph.py

@@ -12,6 +12,7 @@
     MatchValue,
 )
 
+from topix.datatypes.file.document import Document
 from topix.datatypes.graph.graph import Graph
 from topix.datatypes.note.link import Link
 from topix.datatypes.note.note import Note
@@ -122,7 +123,14 @@ def _deep_merge_dict(base: dict, patch: dict) -> dict:
         return merged
 
     async def patch_note(self, node_id: str, data: dict, user_uid: str | None = None) -> Note | None:
-        """Patch a note by merging the update into the full stored note payload."""
+        """Patch a note by merging the update into the full stored note payload.
+
+        Validates the merged payload against the matching pydantic model so
+        documents (subclass of Note with `type: Literal["document"]`) keep
+        their type discriminator and document-specific properties intact —
+        otherwise validating a document row against the bare `Note` model
+        fails with a literal_error on the `type` field.
+        """
         existing_nodes = await self.get_nodes([node_id])
         if not existing_nodes:
             return None
@@ -135,7 +143,8 @@ async def patch_note(self, node_id: str, data: dict, user_uid: str | None = None
             data,
         )
         merged_payload["id"] = node_id
-        merged_note = Note.model_validate(merged_payload)
+        model = Document if isinstance(existing_note, Document) else Note
+        merged_note = model.model_validate(merged_payload)
 
         await self._content_store.update([merged_note.model_dump(exclude_none=False)])
         return merged_note