snap: fix possible crash

- use draw handler to move the expensive synchronous GPU readback
out of the mouse-event loop.
- restore GPU state (depth, blend, face culling) around the offscreen

src/bonsai/bonsai/bim/module/model/decorator.py

@@ -2874,3 +2874,54 @@ class WallSystemPathDecorator(_ConnectedNetworkPathDecorator):
             else:
                 result.append(point)
         return result
+
+
+class GpuSnapDecorator(tool.Blender.ViewportDecorator):
+    """Draw-handler that performs GPU snap detection once per viewport draw.
+
+    The detection work is injected via ``install(..., detection=...)`` so this
+    decorator has no knowledge of the raycast or snap subsystems. The modal
+    snapping code sets the current request via :meth:`set_request` and reads the
+    decoded result from :meth:`get_cache`.
+    """
+
+    draw_method = "draw"
+
+    request: dict[str, Any] = {"mouse_x": 0, "mouse_y": 0, "objs_to_raycast": []}
+    cache: tuple[list[dict], bpy.types.Object | None, list[dict]] | None = None
+    detection: Any = None
+    event: Any = None
+
+    @classmethod
+    def install(cls, context: bpy.types.Context, event=None, detection=None) -> None:
+        if detection is not None:
+            cls.detection = detection
+        cls.event = event
+        cls.request = {"mouse_x": 0, "mouse_y": 0, "objs_to_raycast": []}
+        cls.cache = None
+        super().install(context)
+
+    @classmethod
+    def uninstall(cls) -> None:
+        cls.cache = None
+        cls.event = None
+        super().uninstall()
+
+    @classmethod
+    def set_request(cls, event, objs_to_raycast: list[bpy.types.Object]) -> None:
+        cls.event = event
+        cls.request["mouse_x"] = int(event.mouse_region_x)
+        cls.request["mouse_y"] = int(event.mouse_region_y)
+        cls.request["objs_to_raycast"] = objs_to_raycast
+
+    @classmethod
+    def get_cache(cls) -> tuple[list[dict], bpy.types.Object | None, list[dict]] | None:
+        return cls.cache
+
+    def draw(self, context: bpy.types.Context) -> None:
+        cls = type(self)
+        if cls.detection is None or cls.event is None or not cls.request["objs_to_raycast"]:
+            cls.cache = None
+            return
+
+        cls.cache = cls.detection(context, cls.event, cls.request)

src/bonsai/bonsai/bim/module/model/polyline.py

@@ -26,7 +26,7 @@ import ifcopenshell.util.unit
 from mathutils import Vector
 
 import bonsai.tool as tool
-from bonsai.bim.module.model.decorator import PolylineDecorator
+from bonsai.bim.module.model.decorator import GpuSnapDecorator, PolylineDecorator
 
 
 class PolylineOperator:
@@ -424,6 +424,7 @@ class PolylineOperator:
     def cleanup(self, context: bpy.Types.Context):
         context.workspace.status_text_set(text=None)
         PolylineDecorator.uninstall()
+        GpuSnapDecorator.uninstall()
         tool.Polyline.clear_polyline()
         tool.Raycast.clear_cache()
         tool.Blender.update_viewport()
@@ -460,6 +461,7 @@ class PolylineOperator:
 
     def invoke(self, context: bpy.types.Context, event: bpy.types.Event) -> None:
         PolylineDecorator.install(context)
+        GpuSnapDecorator.install(context, event, detection=tool.Raycast.detect_gpu_snaps)
         tool.Snap.clear_snapping_point()
 
         self.tool_state.use_default_container = False

src/bonsai/bonsai/tool/raycast.py

@@ -1108,6 +1108,13 @@ class Raycast(bonsai.core.tool.Raycast):
         if _offscreen is None:
             _offscreen = GPUOffScreen(max(w, 1), max(h, 1), format="RGBA8")
 
+        # Save GPU state so it can be restored even if readback fails.
+        prev_depth_mask = gpu.state.depth_mask_get()
+        prev_depth_test = gpu.state.depth_test_get()
+        prev_blend = gpu.state.blend_get()
+        face_culling_get = getattr(gpu.state, "face_culling_get", None)
+        prev_face_culling = face_culling_get() if face_culling_get is not None else None
+
         _encoding_shader.bind()
 
         if xray_mode:
@@ -1137,6 +1144,7 @@ class Raycast(bonsai.core.tool.Raycast):
 
         buffers_list = []
         last_buf = None
+        try:
             with _offscreen.bind():
                 fb = gpu.state.active_framebuffer_get()
                 fb.clear(color=(0.0, 0.0, 0.0, 0.0), depth=1.0)
@@ -1154,10 +1162,12 @@ class Raycast(bonsai.core.tool.Raycast):
                         buffers_list.append(buf)
                 if not read_per_object:
                     last_buf = fb.read_color(int(read_x), int(read_y), read_size, read_size, 4, 0, "UBYTE")
-
+        finally:
-        # Restore state
+            gpu.state.depth_mask_set(prev_depth_mask)
-        gpu.state.depth_mask_set(True)
+            gpu.state.depth_test_set(prev_depth_test)
-        gpu.state.depth_test_set("LESS")
+            gpu.state.blend_set(prev_blend)
+            if prev_face_culling is not None:
+                gpu.state.face_culling_set(prev_face_culling)
 
         mouse_read_rect = (w, h, mx, my, read_x, read_y)
         if tris:
@@ -1173,6 +1183,20 @@ class Raycast(bonsai.core.tool.Raycast):
     def get_gpu_wireframe_snaps(cls, context, event, objs_to_raycast):
         return cls.get_gpu_detection_snaps(context, event, objs_to_raycast)
 
+    @classmethod
+    def detect_gpu_snaps(cls, context, event, request):
+        """GPU snap detection callback for ``GpuSnapDecorator``."""
+        if bpy.app.background:
+            return None
+
+        objs_to_raycast = request["objs_to_raycast"]
+        if not objs_to_raycast:
+            return None
+
+        solid_snaps, closest_obj = cls.get_gpu_solid_snaps(context, event, objs_to_raycast)
+        wireframe_snaps, _ = cls.get_gpu_wireframe_snaps(context, event, objs_to_raycast)
+        return solid_snaps, closest_obj, wireframe_snaps
+
     @classmethod
     def clear_cache(cls):
         global _wireframe_batch_cache, _wireframe_vert_fmt, _triangle_batch_cache, _triangle_vert_fmt, _encoding_shader, _offscreen, _obj_list

src/bonsai/bonsai/tool/snap.py

@@ -30,21 +30,21 @@ import bonsai.core.tool
 import bonsai.tool as tool
 from bonsai.bim.module.drawing.data import DecoratorData
 from bonsai.bim.module.drawing.decoration import CutDecorator
-from bonsai.bim.module.model.decorator import PolylineDecorator
+from bonsai.bim.module.model.decorator import GpuSnapDecorator, PolylineDecorator
 
 if TYPE_CHECKING:
     from bonsai.bim.prop import BIMSnapGroups, BIMSnapProperties
 
 
-def _get_gpu_object_snaps(
+def _process_gpu_object_snaps(
     context: bpy.types.Context,
     event: bpy.types.Event,
-    objs_to_raycast: list[bpy.types.Object],
+    snap_faces: list[dict[str, Any]],
+    closest_obj: bpy.types.Object | None,
+    wireframe_snaps: list[dict[str, Any]],
     xray_mode: bool,
 ) -> list[dict[str, Any]]:
     detected_snaps: list[dict[str, Any]] = []
-    snap_faces, closest_obj = tool.Raycast.get_gpu_solid_snaps(context, event, objs_to_raycast)
-    wireframe_snaps, _ = tool.Raycast.get_gpu_wireframe_snaps(context, event, objs_to_raycast)
 
     if not xray_mode:
         for snap in snap_faces:
@@ -85,6 +85,17 @@ def _get_gpu_object_snaps(
     return detected_snaps
 
 
+def _get_gpu_object_snaps(
+    context: bpy.types.Context,
+    event: bpy.types.Event,
+    objs_to_raycast: list[bpy.types.Object],
+    xray_mode: bool,
+) -> list[dict[str, Any]]:
+    snap_faces, closest_obj = tool.Raycast.get_gpu_solid_snaps(context, event, objs_to_raycast)
+    wireframe_snaps, _ = tool.Raycast.get_gpu_wireframe_snaps(context, event, objs_to_raycast)
+    return _process_gpu_object_snaps(context, event, snap_faces, closest_obj, wireframe_snaps, xray_mode)
+
+
 def _get_cpu_object_snaps(
     context: bpy.types.Context,
     event: bpy.types.Event,
@@ -551,7 +562,15 @@ class Snap(bonsai.core.tool.Snap):
         props = cls.get_snap_props()
 
         if props.use_gpu_snapping:
+            GpuSnapDecorator.set_request(event, objs_to_raycast)
+            cached = GpuSnapDecorator.get_cache()
+            if cached is None:
                 detected_snaps.extend(_get_gpu_object_snaps(context, event, objs_to_raycast, xray_mode))
+            else:
+                snap_faces, closest_obj, wireframe_snaps = cached
+                detected_snaps.extend(
+                    _process_gpu_object_snaps(context, event, snap_faces, closest_obj, wireframe_snaps, xray_mode)
+                )
         else:
             detected_snaps.extend(_get_cpu_object_snaps(context, event, objs_to_raycast, xray_mode))
             cut_snaps = _get_cut_object_snaps(context, event)