Add dev-notes for Linked_File_Features branch

Living design note per the docs/dev-notes convention: problem, key
facts (library-per-path reuse, SKIP_SAVE last-used-property retention,
IfcDocumentReference conventions, link matrix math), per-feature design
decisions, commit map, and open test items.

Generated with the assistance of an AI coding tool.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/dev-notes/linked-file-features.md

@@ -0,0 +1,204 @@
+<!-- This file was generated with the assistance of an AI coding tool. -->
+
+# Linked file features — queries, styles, transforms, and multi-linking for linked IFC models
+
+> **Living dev note** for the `Linked_File_Features` branch/PR. Read before working
+> on the feature; append decisions and findings as the PR is refined. This is *not* user
+> documentation — at merge it is removed or its durable parts promoted to code comments.
+> See [README.md](README.md) for the convention (introduced on the
+> `opening-template-on-type` branch; not yet on this branch's base).
+
+## Problem
+
+Linked IFC models (`bim.link_ifc`) had several gaps that made them hard to use as a
+"reference in other trades' models" workflow:
+
+- One shared `.ifc.cache.blend` per IFC file meant the **same file could not be linked
+  twice with different selector queries** — both links showed whichever query was cached
+  first in-session, and whichever was cached last after reopening (Blender reuses one
+  library datablock per path).
+- The selector query was not durably stored anywhere in the host IFC, so save → reopen
+  lost or cross-wired the filter; a scripted `bpy.ops.bim.reload_link()` also wiped it.
+- Linked geometry got **flat diffuse-only materials** — external `.blend` styles
+  (`IfcExternallyDefinedSurfaceStyle`) and per-layer materials (layerset slicing) that
+  the normal import applies were ignored.
+- Moving a linked model required an explicit enable-edit → move → save dance on the
+  active link only, with save/cancel buttons in the panel header.
+- The Explore tool's highlight broke (GPU type errors), drew at the link's *original*
+  location when the link had been moved, and `bim.append_inspected_linked_element`
+  placed appended elements at the original location too.
+
+## Key facts established
+
+- **Cache architecture**: `LoadLink.link_ifc` generates a Python script and runs a
+  background Blender subprocess that executes `bim.load_linked_project` and saves a
+  `.ifc.cache.blend`. The host session then *links* (not appends) the `IfcProject/...`
+  collection from that blend and instances it via an empty (the link "handle").
+  Georeferencing metadata lives in a sidecar `.cache.json`; extracted properties in
+  `.cache.sqlite` (whole file, query-independent — deliberately shared across queries).
+- **Blender reuses an in-session library per path.** Loading the same blend path twice
+  yields the same library/collection. This is what broke multi-query linking with a
+  shared cache filename, and why per-query *filenames* (not cache invalidation) are the
+  fix.
+- **Last-used operator properties** are reused on the next *interactive* invocation
+  (UI button), while scripted `bpy.ops` calls always start from defaults. LoadLink's
+  internal `self.query = link.query` fallback assignment was remembered by Blender and
+  leaked into the next button click (`operator_query='IfcWindow'` for the door link).
+  Any `is_property_set()`-based logic is corrupted the same way. Fix: `SKIP_SAVE` on
+  volatile props. **A GUI-only bug like this is invisible to scripted repro** — both
+  headless and windowed `--python` test runs passed while the manual flow failed.
+- **`IfcDocumentReference`** per link: attribute index 1 (`Identification`) already
+  stores the link's 4×4 transformation (existing Bonsai convention). `Description`
+  (IFC4+; **absent in IFC2X3**) now stores the selector query. One
+  `IfcDocumentInformation` (Scope `LINKED_MODEL`) per file, one reference per link.
+- **Geometry iterator materials**: `material.instance_id()` is the STEP id of the
+  `IfcSurfaceStyle` — or of an `IfcMaterial` when the item has a material but no style,
+  hence the `is_a("IfcSurfaceStyle")` guard when resolving external styles.
+- **External styles**: `IfcExternallyDefinedSurfaceStyle.Location` (`.blend`, relative
+  paths resolve against the *linked* IFC, not the host) + `Identification` in
+  `data_block_type/name` form (e.g. `materials/Brick`), same convention as
+  `bim.activate_external_style`.
+- **Chunk pipeline dedups materials by RGBA color** (`np.unique` on a color array), so
+  style identity must ride along as an extra column to survive — added only for styles
+  that actually resolve to an external material, so plain colored styles dedupe exactly
+  as before.
+- **`slice_layerset_mesh` needs a local-space, per-element mesh** (bisect planes are in
+  object space), which the chunk path can't provide (world-space, many elements per
+  mesh) — hence routing multi-layer elements through the instanced path. Its
+  `dissolve_limit` produces **ngons**, which broke the Explore highlight's
+  triangles-from-`polygon.vertices` assumption downstream.
+- **ID properties round-trip as `IDPropertyArray`**, not plain lists (verified in
+  4.5.7: empty list → flat `IDPropertyArray`; nested lists → list of `IDPropertyArray`
+  items), and `GPUIndexBuf` rejects them — selection geometry must be converted to
+  plain tuples on read.
+- **`scene.ray_cast` returns the hit instance's world matrix** (link empty matrix
+  included). For instanced occurrence objects the object's own local matrix is *not*
+  identity, so resolving the instancing empty must compare against
+  `empty.matrix_world @ obj.matrix_world`, not the empty's matrix alone.
+- **Link matrix math**: the handle empty's matrix is `inv(L) @ T @ G` (L = host local
+  matrix from georef props, T = stored transformation, G = linked model's global
+  matrix from the cache json). The world-space displacement of a moved link is
+  therefore `inv(L) @ T @ L` — no json read needed (`calculate_link_delta_matrix`).
+- **Undo consistency of auto-saved moves**: Blender undo of a handle move fires another
+  depsgraph update, so the handler re-saves the reverted matrix — stored state stays
+  consistent without transactions (a handler can't open one).
+
+## Design
+
+### Per-query caches + query persistence (multi-linking)
+
+`tool.Project.get_link_cache_paths(filepath, query)` appends `.md5(query)[:8]` to the
+cache blend/json names; the empty query keeps the legacy un-suffixed names so existing
+caches stay valid. Every cache-path consumer goes through it — `link_ifc` build and
+invalidation, the subprocess json write, model-origin/georef indicator reads,
+`calculate_link_matrix`, `save_link_transformation`, and the per-link
+selectability/wireframe/visibility toggles (which match collections *by library
+filepath* and would otherwise affect every link of the file at once).
+
+The query persists on each link's `IfcDocumentReference.Description` (written by
+`LinkIfc` and `ReloadLink`); `load_linked_models_from_ifc` restores from it, with a
+legacy-JSON fallback that only applies when the file has a **single** link (with
+several links the shared JSON can't say which link it belonged to). IFC2X3 hosts have
+no `Description` — custom queries are not restorable there (accepted).
+
+`LoadLink`/`ReloadLink` volatile properties are `SKIP_SAVE` (see key facts). Cache
+clearing tolerates a missing blend (a reload with a brand-new query points at a
+not-yet-existing filename).
+
+### External styles + layerset slicing in the linked loader
+
+`LoadLinkedProject.get_external_material(style_id)` resolves a style id → appended
+Blender material from the external `.blend`, cached two ways (per style id; per
+appended data-block, so styles sharing one material don't append duplicates). Appended
+materials get their stale `ifc_definition_id` cleared (the source `.blend` may have
+been authored in a Bonsai session; the id would be misread in the linked file *and*
+in the host once the cache links in). Applied in both loading paths — instanced
+occurrences directly, chunks via the style-id column.
+
+Multi-layer elements (`IfcMaterialLayerSetUsage`, >1 layer) route through the
+instanced path and get `slice_layerset_mesh`, which gained a pluggable
+`style_to_material` resolver (defaults to the old `tool.Ifc.get_object` for the normal
+import) — the linked resolver prefers the external material, falling back to a flat
+diffuse from the style's shading colour. Also fixed there: newly appended layer
+materials are registered in the dedup dict (two layers sharing one style used to
+append it twice).
+
+Trade-off: layered walls become individual instanced objects instead of chunk members;
+meshes shared between elements (same geometry id) bake the slice from the first
+element's layerset usage — same behaviour as the normal importer.
+
+### Reload Link dialog
+
+`bim.reload_link` now exposes File Path (+ browse button), Use Relative Path
+(defaulting to the stored path form), Use Cache (default off = old always-rebuild
+behaviour), the False Origin Mode project props, and Query. A file browser can't open
+from inside a props dialog, so the browse button runs `bim.select_link_filepath`
+(fileselect) which *reopens* the reload dialog with the chosen path, carrying the
+in-progress dialog state through the round trip (op props are baked at draw time).
+Path changes update `link.name`/`filepath` and, with a host IFC, the reference
+`Location` + document name — which is why `ReloadLink` became a `tool.Ifc.Operator`.
+Script calls without arguments preserve all stored link values via `is_property_set`.
+
+### Per-row lock toggle + auto-saved transforms
+
+Link editing moved from the panel header into each list row as a lock/unlock icon:
+unlock (`bim.enable_editing_link`) frees the handle; **any movement is persisted
+immediately** by a `depsgraph_update_post` handler (lazy — ticks without transform
+updates cost ~nothing); lock (`bim.disable_editing_link`) saves and locks.
+`bim.edit_link` and the explicit save step are **removed**; cancel/restore semantics
+no longer exist (undo or move it back). The save math lives in
+`tool.Project.save_link_transformation`. Enable/disable take a `link_index`
+(default −1 = active link) so several links can be edited at once and script calls
+stay compatible.
+
+### Explore tool + append fixes for moved links
+
+- Highlight triangles come from `mesh.calc_loop_triangles()` filtered to the queried
+  element's polygon range (ngon-safe); edges keep `polygon.edge_keys` (no diagonals).
+- `get_selected_geometry` converts the ID-prop round trip to plain tuples (GPU
+  rejects `IDPropertyArray`); TRIS drawing gated on its own data.
+- `QueryLinkedElement` passes the ray-cast instance matrix through;
+  `find_obj_root` compares it against `empty @ obj_local` and falls back to the
+  collection's only instance when no matrix is available (select-by-GUID flow).
+- `bim.append_inspected_linked_element` pre-multiplies the imported object's matrix by
+  `calculate_link_delta_matrix(link)`, matching the link by the queried instance's
+  root empty first (filepath alone is ambiguous with several links per file). The
+  element's IFC placement syncs to the moved location on save — intended.
+
+## Status — implemented (verified in Blender, incl. headless + GUI repro runs)
+
+Six commits on `Linked_File_Features`:
+
+- `0096c0f6a2` reload_link without a query preserves the stored one.
+- `40db55e52d` external styles + layerset slicing for linked models
+  (`project/operator.py`, `tool/loader.py`).
+- `d210d4c814` full Reload Link dialog + `bim.select_link_filepath`.
+- `3dc161f0f2` per-row lock toggle, auto-save handler, `edit_link` removed
+  (`project/operator.py`, `project/ui.py`, `project/__init__.py`, `tool/project.py`).
+- `0571d22855` Explore highlight (ngons, IDPropertyArray), moved-link highlight,
+  append placement (`tool/project.py`, `project/operator.py`, `project/decorator.py`).
+- `c14592ec0a` per-query caches, Description persistence, SKIP_SAVE.
+
+End-to-end verified with a two-links-one-file kit (window/door, distinct queries):
+correct visuals on load, after save → reopen → reload, in both headless and windowed
+Blender.
+
+## Things to test / verify
+
+- **IFC2X3 host**: `Description` doesn't exist — link queries silently not restored on
+  reopen (legacy fallback only for single-link files). Acceptable? Warn?
+- **Relative-path links** (`use_relative_path`) through the whole cycle: cache paths,
+  reference `Location`, reload path change, query restore.
+- Same file linked twice, **both moved differently**: Explore highlight and append
+  placement per instance (root-empty matching), per-link visibility toggles.
+- External styles with **image textures**: paths relative to the style's source
+  `.blend` may not resolve from the cache blend's location (shared limitation with the
+  normal import path).
+- Stale cache orphans: per-query filenames accumulate one blend+json pair per distinct
+  query next to the IFC; nothing auto-deletes them. Cleanup on unlink? Document?
+- Mid-drag auto-save writes the IFC reference outside Bonsai's transaction system —
+  confirm no undo-stack weirdness in longer editing sessions.
+- Layerset slicing on meshes shared by elements with *different* usages (offset/sense)
+  bakes the first element's slice — same as normal import, but worth a look with types.
+- `bim.select_link_filepath` round trip when the reload dialog was opened for a
+  non-active link, and dialog-state carry-over after editing the query *then* browsing.