4 files changed, 38 insertions, 4 deletions
diff --git a/source/blender/blenloader/intern/versioning_280.c b/source/blender/blenloader/intern/versioning_280.c
index 5d73ffe26d5..58643e34922 100644
--- a/source/blender/blenloader/intern/versioning_280.c
+++ b/source/blender/blenloader/intern/versioning_280.c
@@ -1272,6 +1272,22 @@ void do_versions_after_linking_280(Main *bmain, ReportList *UNUSED(reports))
         ma->blend_method = MA_BM_BLEND;
       }
     }
+
+    {
+      /* Update all ruler layers to set new flag. */
+      LISTBASE_FOREACH (Scene *, scene, &bmain->scenes) {
+        bGPdata *gpd = scene->gpd;
+        if (gpd == NULL) {
+          continue;
+        }
+        for (bGPDlayer *gpl = gpd->layers.first; gpl; gpl = gpl->next) {
+          if (STREQ(gpl->info, "RulerData3D")) {
+            gpl->flag |= GP_LAYER_IS_RULER;
+            break;
+          }
+        }
+      }
+    }
   }
 }
 
diff --git a/source/blender/editors/space_view3d/view3d_gizmo_ruler.c b/source/blender/editors/space_view3d/view3d_gizmo_ruler.c
index 5625333d837..a5b7fac624d 100644
--- a/source/blender/editors/space_view3d/view3d_gizmo_ruler.c
+++ b/source/blender/editors/space_view3d/view3d_gizmo_ruler.c
@@ -407,6 +407,17 @@ static bool view3d_ruler_item_mousemove(RulerInfo *ruler_info,
 /** \name Ruler/Grease Pencil Conversion
  * \{ */
 
+/* Helper: Find the layer created as ruler. */
+static bGPDlayer *view3d_ruler_layer_get(bGPdata *gpd)
+{
+  for (bGPDlayer *gpl = gpd->layers.first; gpl; gpl = gpl->next) {
+    if (gpl->flag & GP_LAYER_IS_RULER) {
+      return gpl;
+    }
+  }
+  return NULL;
+}
+
 #define RULER_ID "RulerData3D"
 static bool view3d_ruler_to_gpencil(bContext *C, wmGizmoGroup *gzgroup)
 {
@@ -427,12 +438,12 @@ static bool view3d_ruler_to_gpencil(bContext *C, wmGizmoGroup *gzgroup)
   }
   gpd = scene->gpd;
 
-  gpl = BLI_findstring(&gpd->layers, ruler_name, offsetof(bGPDlayer, info));
+  gpl = view3d_ruler_layer_get(gpd);
   if (gpl == NULL) {
     gpl = BKE_gpencil_layer_addnew(gpd, ruler_name, false);
     copy_v4_v4(gpl->color, U.gpencil_new_layer_col);
     gpl->thickness = 1;
-    gpl->flag |= GP_LAYER_HIDE;
+    gpl->flag |= GP_LAYER_HIDE | GP_LAYER_IS_RULER;
   }
 
   gpf = BKE_gpencil_layer_getframe(gpl, CFRA, GP_GETFRAME_ADD_NEW);
@@ -485,8 +496,7 @@ static bool view3d_ruler_from_gpencil(const bContext *C, wmGizmoGroup *gzgroup)
 
   if (scene->gpd) {
     bGPDlayer *gpl;
-    const char *ruler_name = RULER_ID;
-    gpl = BLI_findstring(&scene->gpd->layers, ruler_name, offsetof(bGPDlayer, info));
+    gpl = view3d_ruler_layer_get(scene->gpd);
     if (gpl) {
       bGPDframe *gpf;
       gpf = BKE_gpencil_layer_getframe(gpl, CFRA, GP_GETFRAME_USE_PREV);
diff --git a/source/blender/makesdna/DNA_gpencil_types.h b/source/blender/makesdna/DNA_gpencil_types.h
index 2dffdf82688..1435d0a64b4 100644
--- a/source/blender/makesdna/DNA_gpencil_types.h
+++ b/source/blender/makesdna/DNA_gpencil_types.h
@@ -401,6 +401,8 @@ typedef enum eGPDlayer_Flag {
   GP_LAYER_USE_MASK = (1 << 13),
   /* Flag used to display in Paint mode only layers with keyframe */
   GP_LAYER_SOLO_MODE = (1 << 4),
+  /* Ruler Layer */
+  GP_LAYER_IS_RULER = (1 << 14),
 } eGPDlayer_Flag;
 
 /* bGPDlayer->onion_flag */
diff --git a/source/blender/makesrna/intern/rna_gpencil.c b/source/blender/makesrna/intern/rna_gpencil.c
index 3ad18fcc7a3..2601600c6ee 100644
--- a/source/blender/makesrna/intern/rna_gpencil.c
+++ b/source/blender/makesrna/intern/rna_gpencil.c
@@ -1457,6 +1457,12 @@ static void rna_def_gpencil_layer(BlenderRNA *brna)
       prop, "Solo Mode", "In Paint mode display only layers with keyframe in current frame");
   RNA_def_property_update(prop, NC_GPENCIL | ND_DATA, "rna_GPencil_update");
 
+  /* Layer is used as Ruler. */
+  prop = RNA_def_property(srna, "is_ruler", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", GP_LAYER_IS_RULER);
+  RNA_def_property_ui_text(prop, "Ruler", "This is a special ruler layer");
+  RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+
   /* exposed as layers.active */
 #  if 0
   prop = RNA_def_property(srna, "active", PROP_BOOLEAN, PROP_NONE);