UI: add Panel.bl_order property to control order of panels for add-ons

This fixes poor Cycles panel ordering, with Freestyle and Custom Properties
appearing at the top.

For most cases order of registration is still the easiest way to control
order and it's recommended to keep using that. This is mainly to solve a few
cases where we want a few built-in panels to appear below add-on panels.

release/scripts/modules/rna_prop_ui.py

@@ -280,6 +280,7 @@ class PropertyPanel:
     """
     bl_label = "Custom Properties"
     bl_options = {'DEFAULT_CLOSED'}
+    bl_order = 1000 # Order panel after all others
 
     @classmethod
     def poll(cls, context):

release/scripts/startup/bl_ui/properties_freestyle.py

@@ -39,6 +39,7 @@ class RenderFreestyleButtonsPanel:
 class RENDER_PT_freestyle(RenderFreestyleButtonsPanel, Panel):
     bl_label = "Freestyle"
     bl_options = {'DEFAULT_CLOSED'}
+    bl_order = 10
     COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE'}
 
     def draw_header(self, context):
@@ -66,6 +67,7 @@ class ViewLayerFreestyleButtonsPanel:
     bl_space_type = 'PROPERTIES'
     bl_region_type = 'WINDOW'
     bl_context = "view_layer"
+    bl_order = 10
     # COMPAT_ENGINES must be defined in each subclass, external engines can add themselves here
 
     @classmethod

release/scripts/startup/bl_ui/properties_material.py

@@ -233,6 +233,7 @@ class MATERIAL_PT_viewport(MaterialButtonsPanel, Panel):
     bl_label = "Viewport Display"
     bl_context = "material"
     bl_options = {'DEFAULT_CLOSED'}
+    bl_order = 10
 
     @classmethod
     def poll(cls, context):

release/scripts/startup/bl_ui/properties_object.py

@@ -212,6 +212,7 @@ class OBJECT_PT_collections(ObjectButtonsPanel, Panel):
 class OBJECT_PT_display(ObjectButtonsPanel, Panel):
     bl_label = "Viewport Display"
     bl_options = {'DEFAULT_CLOSED'}
+    bl_order = 10
 
     def draw(self, context):
         layout = self.layout

release/scripts/startup/bl_ui/properties_render.py

@@ -63,6 +63,7 @@ class RENDER_PT_context(Panel):
 class RENDER_PT_color_management(RenderButtonsPanel, Panel):
     bl_label = "Color Management"
     bl_options = {'DEFAULT_CLOSED'}
+    bl_order = 100
     COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE', 'BLENDER_WORKBENCH'}
 
     def draw(self, context):

release/scripts/startup/bl_ui/properties_world.py

@@ -152,6 +152,7 @@ class EEVEE_WORLD_PT_volume(WorldButtonsPanel, Panel):
 class WORLD_PT_viewport_display(WorldButtonsPanel, Panel):
     bl_label = "Viewport Display"
     bl_options = {'DEFAULT_CLOSED'}
+    bl_order = 10
 
     @classmethod
     def poll(cls, context):

source/blender/blenkernel/BKE_screen.h

@@ -208,6 +208,7 @@ typedef struct PanelType {
   short region_type;
   /* For popovers, 0 for default. */
   int ui_units_x;
+  int order;
 
   int flag;
 

source/blender/makesrna/intern/rna_ui.c

@@ -317,19 +317,21 @@ static StructRNA *rna_Panel_register(Main *bmain,
   pt->draw_header = (have_function[2]) ? panel_draw_header : NULL;
   pt->draw_header_preset = (have_function[3]) ? panel_draw_header_preset : NULL;
 
-  /* XXX use "no header" flag for some ordering of panels until we have real panel ordering */
-  if (pt->flag & PNL_NO_HEADER) {
-    PanelType *pth = art->paneltypes.first;
-    while (pth && pth->flag & PNL_NO_HEADER)
-      pth = pth->next;
+  /* Find position to insert panel based on order. */
+  PanelType *pt_iter = art->paneltypes.last;
 
-    if (pth)
-      BLI_insertlinkbefore(&art->paneltypes, pth, pt);
-    else
-      BLI_addtail(&art->paneltypes, pt);
+  for (; pt_iter; pt_iter = pt_iter->prev) {
+    /* No header has priority. */
+    if ((pt->flag & PNL_NO_HEADER) && !(pt_iter->flag & PNL_NO_HEADER)) {
+      continue;
+    }
+    if (pt_iter->order <= pt->order) {
+      break;
+    }
   }
-  else
-    BLI_addtail(&art->paneltypes, pt);
+
+  /* Insert into list. */
+  BLI_insertlinkafter(&art->paneltypes, pt_iter, pt);
 
   if (parent) {
     pt->parent = parent;
@@ -1347,6 +1349,14 @@ static void rna_def_panel(BlenderRNA *brna)
   RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL);
   RNA_def_property_ui_text(prop, "Units X", "When set, defines popup panel width");
 
+  prop = RNA_def_property(srna, "bl_order", PROP_INT, PROP_UNSIGNED);
+  RNA_def_property_int_sdna(prop, NULL, "type->order");
+  RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL);
+  RNA_def_property_ui_text(
+      prop,
+      "Order",
+      "Panels with lower numbers are default ordered before panels with higher numbers");
+
   prop = RNA_def_property(srna, "use_pin", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, NULL, "flag", PNL_PIN);
   RNA_def_property_ui_text(prop, "Pin", "");