Cleanup: use 'context' to make panels show in their section

All panels were calling poll to draw in their section causing a lot of
repeated boiler plate poll functions.

Also rename 'PreferencePanel' to 'CenterAlignMixIn'
since this is it's purpose.

source/blender/editors/space_userpref/space_userpref.c

@@ -35,6 +35,9 @@
 #include "ED_screen.h"
 #include "ED_space_api.h"
 
+#include "RNA_access.h"
+#include "RNA_enum_types.h"
+
 #include "WM_api.h"
 #include "WM_types.h"
 
@@ -119,9 +122,27 @@ static void userpref_main_region_init(wmWindowManager *wm, ARegion *ar)
   ED_region_panels_init(wm, ar);
 }
 
-static void userpref_main_region_draw(const bContext *C, ARegion *ar)
+static void userpref_main_region_layout(const bContext *C, ARegion *ar)
 {
-  ED_region_panels_ex(C, ar, NULL, U.space_data.section_active, true);
+  char id_lower[64];
+  const char *contexts[2] = {id_lower, NULL};
+
+  /* Avoid duplicating identifiers, use existing RNA enum. */
+  {
+    const EnumPropertyItem *items = rna_enum_preference_section_items;
+    int i = RNA_enum_from_value(items, U.space_data.section_active);
+    /* File is from the future. */
+    if (i == -1) {
+      i = 0;
+    }
+    const char *id = items[i].identifier;
+    BLI_assert(strlen(id) < sizeof(id_lower));
+    STRNCPY(id_lower, id);
+    BLI_str_tolower_ascii(id_lower, strlen(id_lower));
+  }
+
+  ED_region_panels_layout_ex(
+      C, ar, &ar->type->paneltypes, contexts, U.space_data.section_active, true, NULL);
 }
 
 static void userpref_operatortypes(void)
@@ -225,7 +246,8 @@ void ED_spacetype_userpref(void)
   art = MEM_callocN(sizeof(ARegionType), "spacetype userpref region");
   art->regionid = RGN_TYPE_WINDOW;
   art->init = userpref_main_region_init;
-  art->draw = userpref_main_region_draw;
+  art->layout = userpref_main_region_layout;
+  art->draw = ED_region_panels_draw;
   art->listener = userpref_main_region_listener;
   art->keymapflag = ED_KEYMAP_UI;
 

source/blender/makesrna/RNA_enum_types.h

@@ -219,6 +219,7 @@ extern const EnumPropertyItem rna_enum_abc_compression_items[];
 extern const EnumPropertyItem rna_enum_context_mode_items[];
 
 extern const EnumPropertyItem rna_enum_curveprofile_preset_items[];
+extern const EnumPropertyItem rna_enum_preference_section_items[];
 
 /* API calls */
 int rna_node_tree_type_to_enum(struct bNodeTreeType *typeinfo);

source/blender/makesrna/intern/rna_userdef.c

@@ -69,7 +69,7 @@ static const EnumPropertyItem opensubdiv_compute_type_items[] = {
 };
 #endif
 
-static const EnumPropertyItem preference_section_items[] = {
+const EnumPropertyItem rna_enum_preference_section_items[] = {
     {USER_SECTION_INTERFACE, "INTERFACE", 0, "Interface", ""},
     {USER_SECTION_THEME, "THEMES", 0, "Themes", ""},
     {USER_SECTION_VIEWPORT, "VIEWPORT", 0, "Viewport", ""},
@@ -92,6 +92,8 @@ static const EnumPropertyItem preference_section_items[] = {
     {USER_SECTION_SYSTEM, "SYSTEM", 0, "System", ""},
     {USER_SECTION_SAVE_LOAD, "SAVE_LOAD", 0, "Save & Load", ""},
     {USER_SECTION_FILE_PATHS, "FILE_PATHS", 0, "File Paths", ""},
+    {0, "", 0, NULL, NULL},
+    {USER_SECTION_EXPERIMENTAL, "EXPERIMENTAL", 0, "Experimental", ""},
     {0, NULL, 0, NULL, NULL},
 };
 
@@ -468,26 +470,22 @@ static const EnumPropertyItem *rna_UseDef_active_section_itemf(bContext *UNUSED(
 {
   UserDef *userdef = ptr->data;
 
-  if ((userdef->flag & USER_DEVELOPER_UI) == 0) {
+  if ((userdef->flag & USER_DEVELOPER_UI) != 0) {
     *r_free = false;
-    return preference_section_items;
+    return rna_enum_preference_section_items;
   }
 
   EnumPropertyItem *items = NULL;
   int totitem = 0;
 
-  RNA_enum_items_add(&items, &totitem, preference_section_items);
+  for (const EnumPropertyItem *it = rna_enum_preference_section_items; it->identifier != NULL;
-  RNA_enum_item_add_separator(&items, &totitem);
+       it++) {
+    if (it->value == USER_SECTION_EXPERIMENTAL) {
+      continue;
+    }
+    RNA_enum_item_add(&items, &totitem, it);
+  }
 
-  EnumPropertyItem item = {
-      .value = USER_SECTION_EXPERIMENTAL,
-      .name = "Experimental",
-      .identifier = "EXPERIMENTAL",
-      .icon = 0,
-      .description = "",
-  };
-
-  RNA_enum_item_add(&items, &totitem, &item);
   RNA_enum_item_end(&items, &totitem);
 
   *r_free = true;
@@ -5942,7 +5940,7 @@ void RNA_def_userdef(BlenderRNA *brna)
 
   prop = RNA_def_property(srna, "active_section", PROP_ENUM, PROP_NONE);
   RNA_def_property_enum_sdna(prop, NULL, "space_data.section_active");
-  RNA_def_property_enum_items(prop, preference_section_items);
+  RNA_def_property_enum_items(prop, rna_enum_preference_section_items);
   RNA_def_property_enum_funcs(prop, NULL, NULL, "rna_UseDef_active_section_itemf");
   RNA_def_property_ui_text(
       prop, "Active Section", "Active section of the preferences shown in the user interface");