UI: add extensions section (shown when extension repos are enabled)

This is a place-holder by default, the work-in-progress extensions
add-on makes use of this.

scripts/startup/bl_ui/space_userpref.py

@@ -1994,6 +1994,36 @@ class USERPREF_PT_keymap(KeymapPanel, Panel):
         # print("runtime", time.time() - start)
 
 
+# -----------------------------------------------------------------------------
+# Extension Panels
+
+class ExtensionsPanel:
+    bl_space_type = 'PREFERENCES'
+    bl_region_type = 'WINDOW'
+    bl_context = "extensions"
+
+
+class USERPREF_PT_extensions(ExtensionsPanel, Panel):
+    bl_label = "Extensions"
+    bl_options = {'HIDE_HEADER'}
+
+    # NOTE: currently disabled by an add-on when used.
+    unused = True
+
+    @classmethod
+    def poll(cls, _context):
+        return cls.unused
+
+    def draw(self, context):
+        layout = self.layout
+
+        row = layout.row()
+        row.label(text="The add-on to use extensions is disabled! See:")
+        row.operator(
+            "wm.url_open", text="Extension Add-on Repo", icon='URL',
+        ).url = "https://projects.blender.org/ideasman42/bl_ext"
+
+
 # -----------------------------------------------------------------------------
 # Add-On Panels
 
@@ -2272,6 +2302,7 @@ class USERPREF_PT_addons(AddOnPanel, Panel):
 # -----------------------------------------------------------------------------
 # Studio Light Panels
 
+
 class StudioLightPanel:
     bl_space_type = 'PREFERENCES'
     bl_region_type = 'WINDOW'
@@ -2633,6 +2664,8 @@ classes = (
     USERPREF_PT_navigation_fly_walk_gravity,
 
     USERPREF_PT_keymap,
+
+    USERPREF_PT_extensions,
     USERPREF_PT_addons,
 
     USERPREF_PT_studiolight_lights,

source/blender/makesdna/DNA_userdef_types.h

@@ -1070,6 +1070,7 @@ typedef enum eUserPref_Section {
   USER_SECTION_NAVIGATION = 14,
   USER_SECTION_FILE_PATHS = 15,
   USER_SECTION_EXPERIMENTAL = 16,
+  USER_SECTION_EXTENSIONS = 17,
 } eUserPref_Section;
 
 /** #UserDef_SpaceData.flag (State of the user preferences UI). */

source/blender/makesrna/intern/rna_userdef.cc

@@ -56,6 +56,7 @@ const EnumPropertyItem rna_enum_preference_section_items[] = {
     {USER_SECTION_EDITING, "EDITING", 0, "Editing", ""},
     {USER_SECTION_ANIMATION, "ANIMATION", 0, "Animation", ""},
     RNA_ENUM_ITEM_SEPR,
+    {USER_SECTION_EXTENSIONS, "EXTENSIONS", 0, "Extensions", ""},
     {USER_SECTION_ADDONS, "ADDONS", 0, "Add-ons", ""},
 #if 0 /* def WITH_USERDEF_WORKSPACES */
     RNA_ENUM_ITEM_SEPR,
@@ -302,6 +303,9 @@ static void rna_PreferencesExperimental_use_extension_repos_set(PointerRNA * /*p
     U.experimental.use_extension_repos = value;
     BKE_callback_exec_null(bmain, BKE_CB_EVT_EXTENSION_REPOS_UPDATE_POST);
   }
+
+  /* Needed to redraw the preferences window. */
+  WM_main_add_notifier(NC_WINDOW, nullptr);
 }
 
 static void rna_userdef_font_update(Main * /*bmain*/, Scene * /*scene*/, PointerRNA * /*ptr*/)
@@ -601,7 +605,10 @@ static const EnumPropertyItem *rna_UseDef_active_section_itemf(bContext * /*C*/,
 {
   UserDef *userdef = static_cast<UserDef *>(ptr->data);
 
-  if ((userdef->flag & USER_DEVELOPER_UI) != 0) {
+  const bool use_developer_ui = (userdef->flag & USER_DEVELOPER_UI) != 0;
+  const bool use_extension_repos = use_developer_ui && U.experimental.use_extension_repos;
+
+  if (use_developer_ui && use_extension_repos) {
     *r_free = false;
     return rna_enum_preference_section_items;
   }
@@ -613,7 +620,14 @@ static const EnumPropertyItem *rna_UseDef_active_section_itemf(bContext * /*C*/,
        it++)
   {
     if (it->value == USER_SECTION_EXPERIMENTAL) {
-      continue;
+      if (use_developer_ui == false) {
+        continue;
+      }
+    }
+    else if (it->value == USER_SECTION_EXTENSIONS) {
+      if (use_extension_repos == false) {
+        continue;
+      }
     }
     RNA_enum_item_add(&items, &totitem, it);
   }