Initial OP code

@@ -1789,0 +1867,4 @@
+      curves_num += c_src + c_dst;
+
+      /* got an Assert with this line. */
+      curves_dst.resize(points_num, curves_num);

@@ -1605,0 +1645,4 @@
+
+  /* Iterate through all the drawings at current scene frame. */
+  const Array<MutableDrawingInfo> drawings_src = retrieve_editable_drawings(*scene, grease_pencil);
+  for (const MutableDrawingInfo &info : drawings_src) {

@@ -1605,0 +1653,4 @@
+      continue;
+    }
+
+    if (grease_pencil.get_drawing_at(*layer_dst, info.frame_number) == nullptr) {

@@ -1605,0 +1658,4 @@
+      grease_pencil.insert_blank_frame(*layer_dst, info.frame_number, 0, BEZT_KEYTYPE_KEYFRAME);
+      /* Copy strokes to new CurvesGeometry. */
+      Drawing &drawing_dst = *grease_pencil.get_editable_drawing_at(*layer_dst, info.frame_number);
+      drawing_dst.strokes_for_write() = bke::curves_copy_point_selection(

@@ -1605,0 +1612,4 @@
+static int grease_pencil_move_to_layer_exec(bContext *C, wmOperator *op)
+{
+  using namespace bke::greasepencil;
+  Scene *scene = CTX_data_scene(C);

@@ -1605,0 +1618,4 @@
+  Object *object = CTX_data_active_object(C);
+  GreasePencil &grease_pencil = *static_cast<GreasePencil *>(object->data);
+  Layer *layer_dst = nullptr;
+  int layer_index = RNA_int_get(op->ptr, "layer");

@@ -277,0 +282,4 @@
+        layout.operator_context = 'INVOKE_REGION_WIN'
+        obd = context.active_object.data
+
+        layout.operator("grease_pencil.move_to_layer", text="New Layer", icon='ADD').layer = -1        

@@ -277,0 +280,4 @@
+    def draw(self, context):
+        layout = self.layout
+        layout.operator_context = 'INVOKE_REGION_WIN'
+        obd = context.active_object.data

@@ -277,0 +282,4 @@
+        layout.operator_context = 'INVOKE_REGION_WIN'
+        obd = context.active_object.data
+        
+        nlop = layout.operator("grease_pencil.layer_add", text="New Layer", icon='ADD')

@@ -13,6 +13,7 @@
 #include "BLI_math_vector_types.hh"
 #include "BLI_span.hh"
 #include "BLI_stack.hh"
+#include "BLI_string.h"

@@ -41,3 +43,4 @@
 #include "WM_api.hh"
 
 #include "UI_resources.hh"
+using namespace blender::bke;

@@ -1605,0 +1624,4 @@
+    /* get layer by index */
+    layer_dst = grease_pencil.layers_for_write()[layer_index];
+  }
+  else {

@@ -16,6 +16,8 @@
 
 #include "ED_keyframes_edit.hh"
 
+#include "WM_api.hh"

@@ -277,0 +281,4 @@
+        layout = self.layout
+        layout.operator_context = 'INVOKE_REGION_WIN'
+        grease_pencil = context.active_object.data        
+        

@@ -277,0 +283,4 @@
+        grease_pencil = context.active_object.data        
+        
+        layout.operator("grease_pencil.layer_add", text="New Layer", icon='ADD').new_layer_name = "Layer"        
+        

@@ -1605,0 +1650,4 @@
+      drawing_dst.tag_topology_changed();
+    }
+    else {
+      /* For existing Layers append the strokes to new CurvesGeometry. */

@@ -1605,0 +1669,4 @@
+
+    info.drawing.tag_topology_changed();
+    changed = true;
+  };

@@ -1605,0 +1638,4 @@
+    }
+
+    if (!layer_dst->has_drawing_at(info.frame_number)) {
+      /* For new layers Insert Keyframe at current frame/layer. */

@@ -1605,0 +1691,4 @@
+
+  /* callbacks. */
+  ot->exec = grease_pencil_move_to_layer_exec;
+  ot->poll = ot->poll = editable_grease_pencil_poll;

@@ -1605,0 +1641,4 @@
+      /* For new layers Insert Keyframe at current frame/layer. */
+      grease_pencil.insert_blank_frame(*layer_dst, info.frame_number, 0, BEZT_KEYTYPE_KEYFRAME);
+      /* Copy strokes to new CurvesGeometry. */
+      Drawing &drawing_dst = *grease_pencil.get_editable_drawing_at(*layer_dst, info.frame_number);

@@ -1605,0 +1651,4 @@
+    }
+    else {
+      /* For existing Layers append the strokes to new CurvesGeometry. */
+      Drawing &drawing_dst = *grease_pencil.get_editable_drawing_at(*layer_dst, info.frame_number);

@@ -1605,0 +1699,4 @@
+  /* Grease Pencil layer to use. */
+  prop = RNA_def_int(ot->srna, "layer", 0, -1, INT_MAX, "Grease Pencil Layer", "", -1, INT_MAX);
+  RNA_def_property_flag(prop, PROP_HIDDEN | PROP_SKIP_SAVE);
+  RNA_def_string(

@@ -1605,0 +1697,4 @@
+  ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
+
+  /* Grease Pencil layer to use. */
+  prop = RNA_def_int(ot->srna, "layer", 0, -1, INT_MAX, "Grease Pencil Layer", "", -1, INT_MAX);