Added back the extrude button to mdr ed's sidebar, and implemented an extrude algorithm. I have high hopes for it, however it does not work at the moment, because SurfaceTool messes up the order of the triangles in the mesh after a normal generation, which I will fix soon.

game/addons/mesh_data_resource_editor/MDIEd.gd

@@ -147,7 +147,7 @@ func set_selection_mode_face() -> void:
 
 
 func _on_Extrude_pressed():
-	pass # Replace with function body.
+	_plugin.extrude()
 
 func _on_AddBox_pressed():
 	_plugin.add_box()

game/addons/mesh_data_resource_editor/MDIEd.tscn

@@ -203,7 +203,7 @@ scroll_horizontal_enabled = false
 
 [node name="VBoxContainer2" type="VBoxContainer" parent="VBoxContainer/ScrollContainer"]
 margin_right = 998.0
-margin_bottom = 690.0
+margin_bottom = 698.0
 size_flags_horizontal = 3
 
 [node name="HBoxContainer6" type="HBoxContainer" parent="VBoxContainer/ScrollContainer/VBoxContainer2"]
@@ -459,6 +459,11 @@ margin_top = 18.0
 margin_right = 998.0
 margin_bottom = 98.0
 
+[node name="Extrude" type="Button" parent="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations"]
+margin_right = 1010.0
+margin_bottom = 20.0
+text = "Extrude"
+
 [node name="Label" type="Label" parent="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations"]
 margin_right = 998.0
 margin_bottom = 14.0
@@ -485,12 +490,6 @@ margin_bottom = 20.0
 size_flags_horizontal = 3
 text = "Quad"
 
-[node name="Extrude" type="Button" parent="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations"]
-visible = false
-margin_right = 1010.0
-margin_bottom = 20.0
-text = "Extrude"
-
 [node name="SeamLabel" type="Label" parent="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations"]
 margin_top = 42.0
 margin_right = 998.0
@@ -744,9 +743,9 @@ size_flags_vertical = 3
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/VertexOps/Operations/HBoxContainer/ConnectToFirst" to="." method="_on_connect_to_first_selected_pressed"]
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/VertexOps/Operations/HBoxContainer/ConnectMed" to="." method="_on_connect_to_avg_pressed"]
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/VertexOps/Operations/HBoxContainer/ConnectToLast" to="." method="_on_connect_to_last_selected_pressed"]
+[connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations/Extrude" to="." method="_on_Extrude_pressed"]
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations/HBoxContainer2/TriAt" to="." method="_on_add_triangle_at_pressed"]
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations/HBoxContainer2/QuadAt" to="." method="_on_add_auad_at_pressed"]
-[connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations/Extrude" to="." method="_on_Extrude_pressed"]
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations/HBoxContainer/Mark" to="." method="_on_mark_seam_pressed"]
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/EdgeOps/Operations/HBoxContainer/Unmark" to="." method="_on_unmark_seam_pressed"]
 [connection signal="pressed" from="VBoxContainer/ScrollContainer/VBoxContainer2/FaceOps/Operations/Delete" to="." method="_on_delete_pressed"]

game/addons/mesh_data_resource_editor/MIDGizmo.gd

@@ -939,7 +939,92 @@ func add_quad_at() -> void:
 		add_mesh_change_undo_redo(orig_arr, _mdr.array, "Add Quad At")
 		enable_change_event()
 	else:
-		add_triangle()
+		add_quad()
+
+func extrude() -> void:
+	if !_mdr:
+		return
+	
+	if _mdr.array.size() != ArrayMesh.ARRAY_MAX || _mdr.array[ArrayMesh.ARRAY_VERTEX] == null:
+		return
+	
+	if selection_mode == SelectionMode.SELECTION_MODE_VERTEX:
+		pass
+	elif selection_mode == SelectionMode.SELECTION_MODE_EDGE:
+		disable_change_event()
+		var orig_arr = copy_arrays(_mdr.array)
+		var original_size : int = orig_arr[ArrayMesh.ARRAY_VERTEX].size()
+		
+		for sp in _selected_points:
+			add_quad_to_edge(sp)
+
+		var arr : Array = _mdr.array
+		
+		# Note: This algorithm depends heavily depends on the inner workings of add_quad_to_edge!
+		var new_verts : PoolVector3Array = arr[ArrayMesh.ARRAY_VERTEX]
+		
+		# every 4 vertex is a quad
+		# 1 ---- 2
+		# |      |
+		# |      |
+		# 0 ---- 3
+		# vertex 1, and 2 are the created new ones, 0, and 3 are duplicated from the original edge
+		
+		# Don't reallocate it every time
+		var found_verts : PoolIntArray = PoolIntArray()
+		
+		# Go through every new created 0th vertex
+		for i in range(original_size, new_verts.size(), 4):
+			var v0 : Vector3 = new_verts[i]
+			
+			found_verts.resize(0)
+			
+			# Find a pair for it (has to be the 3th).
+			for j in range(original_size, new_verts.size(), 4):
+				if i == j:
+					continue
+				
+				# +3 offset to 3rd vert
+				var v3 : Vector3 = new_verts[j + 3]
+
+				if is_verts_equal(v0, v3):
+					# +2 offset to 2rd vert
+					found_verts.append(j + 2)
+			
+			if found_verts.size() == 0:
+				continue
+			
+			# Also append the first vertex index to simplify logic
+			found_verts.append(i + 1)
+			
+			# Calculate avg
+			var vavg : Vector3 = Vector3()
+			for ind in found_verts:
+				vavg += new_verts[ind]
+				
+			vavg /= found_verts.size()
+			
+			# set back
+			for ind in found_verts:
+				new_verts[ind] = vavg
+
+		arr[ArrayMesh.ARRAY_VERTEX] = new_verts
+		_mdr.array = arr
+		
+		_selected_points.resize(0)
+		add_mesh_change_undo_redo(orig_arr, _mdr.array, "Extrude")
+		enable_change_event()
+		
+		# The selection alo will take care of the duplicates
+		var new_handle_points : PoolVector3Array = PoolVector3Array()
+		for i in range(original_size, new_verts.size() - 4, 4):
+			new_handle_points.append(new_verts[i + 1])
+			new_handle_points.append(new_verts[i + 2])
+		
+		# select new ones
+		# TODO
+	else:
+		add_quad()
 
 func add_box() -> void:
 	if _mdr:

game/addons/mesh_data_resource_editor/plugin.gd

@@ -272,3 +272,7 @@ func handle_selection_type_back():
 func handle_selection_type_all():
 	if current_mesh_data_instance && current_mesh_data_instance.gizmo:
 		current_mesh_data_instance.gizmo.handle_selection_type_all()
+
+func extrude():
+	if current_mesh_data_instance && current_mesh_data_instance.gizmo:
+		current_mesh_data_instance.gizmo.extrude()