UI: Warn when using fallback category

Scripts should be updated.

source/blender/editors/interface/interface_panel.c

@@ -1534,6 +1534,12 @@ void UI_panel_category_draw_all(ARegion *ar, const char *category_id_active)
 
 		const bool is_active = STREQ(category_id, category_id_active);
 
+#ifdef DEBUG
+		if (STREQ(category_id, PNL_CATEGORY_FALLBACK)) {
+			printf("WARNING: Panel has no 'bl_category', script needs updating!\n");
+		}
+#endif
+
 		glEnable(GL_BLEND);
 
 #ifdef USE_FLAT_INACTIVE

source/blender/makesdna/DNA_screen_types.h

@@ -315,8 +315,8 @@ enum {
 #define PNL_DEFAULT_CLOSED		1
 #define PNL_NO_HEADER			2
 
-/* Default panel category. */
-#define PNL_DEFAULT_CATEGORY "Misc"
+/* Fallback panel category (only for old scripts which need updating) */
+#define PNL_CATEGORY_FALLBACK "Misc"
 
 /* uiList layout_type */
 enum {

source/blender/makesrna/intern/rna_ui.c

@@ -201,9 +201,6 @@ static StructRNA *rna_Panel_register(Main *bmain, ReportList *reports, void *dat
 	/* We have to set default context! Else we get a void string... */
 	strcpy(dummypt.translation_context, BLF_I18NCONTEXT_DEFAULT_BPYRNA);
 
-	/* We have to set default category! Else we get a void string, and panel goes everywhere! */
-	strcpy(dummypt.category, PNL_DEFAULT_CATEGORY);
-
 	/* validate the python class */
 	if (validate(&dummyptr, data, have_function) != 0)
 		return NULL;
@@ -214,6 +211,11 @@ static StructRNA *rna_Panel_register(Main *bmain, ReportList *reports, void *dat
 		return NULL;
 	}
 
+	if ((dummypt.category[0] == '\0') && (dummypt.region_type == RGN_TYPE_TOOLS)) {
+		/* Use a fallback, otherwise an empty value will draw the panel in every category. */
+		strcpy(dummypt.category, PNL_CATEGORY_FALLBACK);
+	}
+
 	if (!(art = region_type_find(reports, dummypt.space_type, dummypt.region_type)))
 		return NULL;
 
@@ -990,7 +992,6 @@ static void rna_def_panel(BlenderRNA *brna)
 
 	prop = RNA_def_property(srna, "bl_category", PROP_STRING, PROP_NONE);
 	RNA_def_property_string_sdna(prop, NULL, "type->category");
-	RNA_def_property_string_default(prop, PNL_DEFAULT_CATEGORY);
 	RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL);
 
 	prop = RNA_def_property(srna, "bl_space_type", PROP_ENUM, PROP_NONE);