Add extend/regenerate walls to multiple undersides

extend_walls_to_underside now accepts multiple slab/roof
objects in a single operation — all selected non-LAYER2 IFC
elements are treated as clip targets, all LAYER2 elements as
walls. Placement sync is done once upfront; each wall is then
clipped against every selected slab before reloading.

Also adds bim.regenerate_wall_to_underside (Shift+G): after
moving a slab, re-clips connected walls using the existing
IfcRelConnectsElements(TOP) relationship. Old booleans are
removed via remove_representation_item before re-clipping.

Generated with the assistance of an AI coding tool.

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

@@ -300,20 +300,21 @@ class ExtendWallsToUnderside(bpy.types.Operator, tool.Ifc.Operator):
         # of the selected walls has an in-progress parametric draft, commit it before
         # extending, so the slab clip operates on the just-finalised IFC state.
         _commit_pending_wall_edits_for_selection(context)
-        slab = None
+        slabs: list[bpy.types.Object] = []
         walls: list[bpy.types.Object] = []
-        if (obj := tool.Blender.get_active_object(is_selected=True)) and (element := tool.Ifc.get_entity(obj)):
-            slab = obj
-        for obj in tool.Blender.get_selected_objects(include_active=False):
+        for obj in tool.Blender.get_selected_objects():
             element = tool.Ifc.get_entity(obj)
-            usage = tool.Model.get_usage_type(element) if element else None
-            if element and usage == "LAYER2":
+            if not element:
+                continue
+            if tool.Model.get_usage_type(element) == "LAYER2":
                 walls.append(obj)
-        if slab and walls:
-            core.extend_wall_to_slab(tool.Ifc, tool.Geometry, tool.Model, slab, walls)
+            else:
+                slabs.append(obj)
+        if slabs and walls:
+            core.extend_wall_to_slab(tool.Ifc, tool.Geometry, tool.Model, slabs, walls)
             _resync_walls_after_mutation(walls)
         else:
-            self.report({"ERROR"}, "Please select at least one LAYER2 element and an active element")
+            self.report({"ERROR"}, "Please select at least one LAYER2 element and at least one other IFC element")
 
 
 class RegenerateWallToUnderside(bpy.types.Operator, tool.Ifc.Operator):

src/bonsai/bonsai/core/model.py

@@ -190,20 +190,26 @@ def extend_wall_to_slab(
     ifc: type[tool.Ifc],
     geometry: type[tool.Geometry],
     model: type[tool.Model],
-    slab_obj: bpy.types.Object,
+    slab_objs: list[bpy.types.Object],
     wall_objs: list[bpy.types.Object],
 ) -> None:
-    clip = model.get_slab_clipping_bmesh(slab_obj)
-    if not clip:
-        return
-    slab = ifc.get_entity(slab_obj)
     for obj in wall_objs:
         if ifc.is_moved(obj):
             geometry.run_edit_object_placement(obj=obj)
-        wall = ifc.get_entity(obj)
-        model.clip_wall_to_slab(wall, clip)
-        model.connect_wall_to_slab(wall, slab)
-    model.reload_body_representation(wall_objs)
+    clipped_walls = []
+    for slab_obj in slab_objs:
+        clip = model.get_slab_clipping_bmesh(slab_obj)
+        if not clip:
+            continue
+        slab = ifc.get_entity(slab_obj)
+        for obj in wall_objs:
+            wall = ifc.get_entity(obj)
+            model.clip_wall_to_slab(wall, clip)
+            model.connect_wall_to_slab(wall, slab)
+            if obj not in clipped_walls:
+                clipped_walls.append(obj)
+    if clipped_walls:
+        model.reload_body_representation(clipped_walls)
 
 
 class RequireTwoWallsError(Exception):