Fix T96338: GPU subdiv crash switching to UV editing

The crash is caused as the data for the UV editor is requested before
the data for the mesh as a separate draw update. Since building the UV
stretch angle buffer requires the position buffer, the latter is not
created yet in this case.

To fix this, create a local position buffer from the subdivision data. An
alternate fix was considered to remove the dependency on the position
buffer by interpolating on the GPU the coarse stretch angle buffer but
this did work. Maybe this will be revisited.

source/blender/draw/intern/draw_cache_impl_subdivision.cc

@@ -366,6 +366,17 @@ static GPUVertFormat *get_origindex_format()
   return &format;
 }
 
+GPUVertFormat *draw_subdiv_get_pos_nor_format()
+{
+  static GPUVertFormat format = {0};
+  if (format.attr_len == 0) {
+    GPU_vertformat_attr_add(&format, "pos", GPU_COMP_F32, 3, GPU_FETCH_FLOAT);
+    GPU_vertformat_attr_add(&format, "nor", GPU_COMP_F32, 4, GPU_FETCH_FLOAT);
+    GPU_vertformat_alias_add(&format, "vnor");
+  }
+  return &format;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/draw/intern/draw_subdivision.h

@@ -13,6 +13,7 @@ struct BMesh;
 struct GPUIndexBuf;
 struct GPUUniformBuf;
 struct GPUVertBuf;
+struct GPUVertFormat;
 struct Mesh;
 struct MeshBatchCache;
 struct MeshBufferCache;
@@ -284,6 +285,10 @@ void draw_subdiv_build_edituv_stretch_angle_buffer(const DRWSubdivCache *cache,
                                                    int uvs_offset,
                                                    struct GPUVertBuf *stretch_angles);
 
+/** Return the format used for the positions and normals VBO.
+ */
+struct GPUVertFormat *draw_subdiv_get_pos_nor_format();
+
 #ifdef __cplusplus
 }
 #endif

source/blender/draw/intern/mesh_extractors/extract_mesh_vbo_edituv_stretch_angle.cc

@@ -224,6 +224,21 @@ static void extract_edituv_stretch_angle_init_subdiv(const DRWSubdivCache *subdi
   GPUVertBuf *pos_nor = cache->final.buff.vbo.pos_nor;
   GPUVertBuf *uvs = cache->final.buff.vbo.uv;
 
+  /* It may happen that the data for the UV editor is requested before (as a separate draw update)
+   * the data for the mesh when switching to the `UV Editing` workspace, and therefore the position
+   * buffer might not be created yet. In this case, create a buffer it locally, the subdivision
+   * data should already be evaluated if we are here. This can happen if the subsurf modifier is
+   * only enabled in edit-mode. See T96338. */
+  if (!pos_nor) {
+    const DRWSubdivLooseGeom &loose_geom = subdiv_cache->loose_geom;
+    pos_nor = GPU_vertbuf_calloc();
+    GPU_vertbuf_init_build_on_device(pos_nor,
+                                     draw_subdiv_get_pos_nor_format(),
+                                     subdiv_cache->num_subdiv_loops + loose_geom.loop_len);
+
+    draw_subdiv_extract_pos_nor(subdiv_cache, pos_nor);
+  }
+
   /* UVs are stored contiguously so we need to compute the offset in the UVs buffer for the active
    * UV layer. */
   CustomData *cd_ldata = (mr->extract_type == MR_EXTRACT_MESH) ? &mr->me->ldata : &mr->bm->ldata;
@@ -253,6 +268,10 @@ static void extract_edituv_stretch_angle_init_subdiv(const DRWSubdivCache *subdi
 
   draw_subdiv_build_edituv_stretch_angle_buffer(
       subdiv_cache, pos_nor, uvs, uvs_offset, refined_vbo);
+
+  if (!cache->final.buff.vbo.pos_nor) {
+    GPU_vertbuf_discard(pos_nor);
+  }
 }
 
 constexpr MeshExtract create_extractor_edituv_edituv_stretch_angle()

source/blender/draw/intern/mesh_extractors/extract_mesh_vbo_pos_nor.cc

@@ -179,17 +179,6 @@ static void extract_pos_nor_finish(const MeshRenderData *UNUSED(mr),
   MEM_freeN(data->normals);
 }
 
-static GPUVertFormat *get_pos_nor_format()
-{
-  static GPUVertFormat format = {0};
-  if (format.attr_len == 0) {
-    GPU_vertformat_attr_add(&format, "pos", GPU_COMP_F32, 3, GPU_FETCH_FLOAT);
-    GPU_vertformat_attr_add(&format, "nor", GPU_COMP_F32, 4, GPU_FETCH_FLOAT);
-    GPU_vertformat_alias_add(&format, "vnor");
-  }
-  return &format;
-}
-
 static GPUVertFormat *get_normals_format()
 {
   static GPUVertFormat format = {0};
@@ -221,7 +210,7 @@ static void extract_pos_nor_init_subdiv(const DRWSubdivCache *subdiv_cache,
 
   /* Initialize the vertex buffer, it was already allocated. */
   GPU_vertbuf_init_build_on_device(
-      vbo, get_pos_nor_format(), subdiv_cache->num_subdiv_loops + loose_geom.loop_len);
+      vbo, draw_subdiv_get_pos_nor_format(), subdiv_cache->num_subdiv_loops + loose_geom.loop_len);
 
   if (subdiv_cache->num_subdiv_loops == 0) {
     return;