decoration: bring back border_part_of_window

fixes #9683, now under decoration: though

src/config/ConfigDescriptions.hpp

@@ -247,6 +247,12 @@ inline static const std::vector<SConfigOptionDescription> CONFIG_OPTIONS = {
         .type        = CONFIG_OPTION_STRING_LONG,
         .data        = SConfigOptionDescription::SStringData{""}, //##TODO UNSET?
     },
+    SConfigOptionDescription{
+        .value       = "decoration:border_part_of_window",
+        .description = "whether the border should be treated as a part of the window.",
+        .type        = CONFIG_OPTION_BOOL,
+        .data        = SConfigOptionDescription::SBoolData{true},
+    },
 
     /*
      * blur: