Stop the panels building collections while they draw

get_products() reached the MV_Products collection through
get_products_root(), which creates and links it when it is missing. Three
panels call that from draw(), so merely opening the MultiView sidebar on
a file that had never used the add-on linked an MV_Products collection
into the scene and left the file marked as modified.

Blender does not refuse the write, it just does it, and the resulting
data change schedules another redraw: with the sidebar open the panels
drew twice per change instead of once. Building data from a draw callback
is not something the API promises to keep working either.

get_products() now reads bpy.data directly and returns [] when the root
is absent. Creation stays in get_products_root(), which only the import
and add-collection operators call. Every caller already handled an empty
list, since that is what an existing but empty MV_Products returns.

Author: Danyals-code

multiview_product_renderer/__init__.py

@@ -313,11 +313,23 @@ def ensure_collection(name, parent=None):
 
 
 def get_products_root():
+    """The MV_Products collection, created if it is not there yet.
+
+    Only for operators. Panels read the products with get_products(), which
+    never creates anything: draw() runs on every redraw, so reaching for the
+    root here linked an MV_Products collection into the scene merely because
+    someone opened the sidebar, and left their file marked as modified for it.
+    """
     return ensure_collection(PRODUCT_COLL)
 
 
 def get_products():
-    return list(get_products_root().children)
+    """Product collections, or [] when the root does not exist yet.
+
+    Read-only on purpose -- see get_products_root().
+    """
+    root = bpy.data.collections.get(PRODUCT_COLL)
+    return list(root.children) if root else []
 
 
 def bbox_of(objs):
@@ -1826,6 +1838,7 @@ def draw(self, context):
                 for pc in prods:
                     box.label(text=f"{pc.name}  ({len(pc.all_objects)} objs)", icon='OUTLINER_COLLECTION')
 
+
 class MV_PT_Standardize(Panel):
     bl_label = "Standardize"
     bl_space_type = 'VIEW_3D'